[javascript] How to update and delete a cookie?

I need help to know how to update values and how to delete a cookie created from this code! I'm new to JavaScript so it's great if anyone can help me.

function getCookie(c_name) {
     var i,x,y,ARRcookies = document.cookie.split(";");
     for (i=0;i<ARRcookies.length;i++) {
          x = ARRcookies[i].substr(0,ARRcookies[i].indexOf("="));
          y = ARRcookies[i].substr(ARRcookies[i].indexOf("=")+1);
          x = x.replace(/^\s+|\s+$/g,"");
          if (x==c_name) {
              return unescape(y);
          }
      }
}

function setCookie(c_name,value,exdays) {
    var exdate = new Date();
    exdate.setDate(exdate.getDate() + exdays);
    var c_value = escape(value) + ((exdays==null) ? "" : ";                                    expires="+exdate.toUTCString());
   document.cookie=c_name + "=" + c_value;
 }

function check2Cookie() {
     var username=getCookie("username");
     if (username!=null && username!="") {
         username= "0";
         setCookie("username",username,1000);
     }
     else {
         username=" ";
         if (username!=null && username!="") {
               username= "0";
               setCookie("username",username,1000);
         }
     }
}

This is the code for cookie creation.

Code for creating is setCookie("username",username,1000);

Now how to update this cookie and delete this cookie.

check this out A little framework: a complete cookies reader/writer with full Unicode support

/*\
|*|
|*|  :: cookies.js ::
|*|
|*|  A complete cookies reader/writer framework with full unicode support.
|*|
|*|  Revision #1 - September 4, 2014
|*|
|*|  https://developer.mozilla.org/en-US/docs/Web/API/document.cookie
|*|  https://developer.mozilla.org/User:fusionchess
|*|  https://github.com/madmurphy/cookies.js
|*|
|*|  This framework is released under the GNU Public License, version 3 or later.
|*|  http://www.gnu.org/licenses/gpl-3.0-standalone.html
|*|
|*|  Syntaxes:
|*|
|*|  * docCookies.setItem(name, value[, end[, path[, domain[, secure]]]])
|*|  * docCookies.getItem(name)
|*|  * docCookies.removeItem(name[, path[, domain]])
|*|  * docCookies.hasItem(name)
|*|  * docCookies.keys()
|*|
\*/

var docCookies = {
  getItem: function (sKey) {
    if (!sKey) { return null; }
    return decodeURIComponent(document.cookie.replace(new RegExp("(?:(?:^|.*;)\\s*" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=\\s*([^;]*).*$)|^.*$"), "$1")) || null;
  },
  setItem: function (sKey, sValue, vEnd, sPath, sDomain, bSecure) {
    if (!sKey || /^(?:expires|max\-age|path|domain|secure)$/i.test(sKey)) { return false; }
    var sExpires = "";
    if (vEnd) {
      switch (vEnd.constructor) {
        case Number:
          sExpires = vEnd === Infinity ? "; expires=Fri, 31 Dec 9999 23:59:59 GMT" : "; max-age=" + vEnd;
          break;
        case String:
          sExpires = "; expires=" + vEnd;
          break;
        case Date:
          sExpires = "; expires=" + vEnd.toUTCString();
          break;
      }
    }
    document.cookie = encodeURIComponent(sKey) + "=" + encodeURIComponent(sValue) + sExpires + (sDomain ? "; domain=" + sDomain : "") + (sPath ? "; path=" + sPath : "") + (bSecure ? "; secure" : "");
    return true;
  },
  removeItem: function (sKey, sPath, sDomain) {
    if (!this.hasItem(sKey)) { return false; }
    document.cookie = encodeURIComponent(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT" + (sDomain ? "; domain=" + sDomain : "") + (sPath ? "; path=" + sPath : "");
    return true;
  },
  hasItem: function (sKey) {
    if (!sKey) { return false; }
    return (new RegExp("(?:^|;\\s*)" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=")).test(document.cookie);
  },
  keys: function () {
    var aKeys = document.cookie.replace(/((?:^|\s*;)[^\=]+)(?=;|$)|^\s*|\s*(?:\=[^;]*)?(?:\1|$)/g, "").split(/\s*(?:\=[^;]*)?;\s*/);
    for (var nLen = aKeys.length, nIdx = 0; nIdx < nLen; nIdx++) { aKeys[nIdx] = decodeURIComponent(aKeys[nIdx]); }
    return aKeys;
  }
};

http://www.quirksmode.org/js/cookies.html

function createCookie(name,value,days) {
    if (days) {
        var date = new Date();
        date.setTime(date.getTime()+(days*24*60*60*1000));
        var expires = "; expires="+date.toGMTString();
    }
    else var expires = "";
    document.cookie = name+"="+value+expires+"; path=/";
}

function readCookie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
    }
    return null;
}

function eraseCookie(name) {
    createCookie(name,"",-1);
}

The cookie API is kind of lame. Let me clarify...

You don't update cookies; you overwrite them:

    document.cookie = "username=Arnold"; // Create 'username' cookie
    document.cookie = "username=Chuck"; // Update, i.e. overwrite, the 'username' cookie to "Chuck"

You also don't delete cookies; you expire them by setting the expires key to a time in the past (-1 works too).

Source: https://developer.mozilla.org/en-US/docs/Web/API/document.cookie