Since you are disabling it in the first place, the way to enable it is to set its disabled property as false.
To change its disabled property in Javascript, you use this:
var btn = document.getElementById("Button");
btn.disabled = false;
And obviously to disable it again, you'd use true instead.
Since you also tagged the question with jQuery, you could use the .prop method. Something like:
var btn = $("#Button");
btn.prop("disabled", true); // Or `false`
This is in the newer versions of jQuery. The older way to do this is to add or remove an attribute like so:
var btn = $("#Button");
btn.attr("disabled", "disabled");
// or
btn.removeAttr("disabled");
The mere presence of the disabled property disables the element, so you cannot set its value as "false". Even the following should disable the element
<input type="button" value="Submit" disabled="" />
You need to either remove the attribute completely or set its property.