$mylabel.text( $mylabel.text().replace('-', '') );
Since text() gets the value, and text( "someValue" ) sets the value, you just place one inside the other.
Would be the equivalent of doing:
var newValue = $mylabel.text().replace('-', '');
$mylabel.text( newValue );
EDIT:
I hope I understood the question correctly. I'm assuming $mylabel is referencing a DOM element in a jQuery object, and the string is in the content of the element.
If the string is in some other variable not part of the DOM, then you would likely want to call the .replace() function against that variable before you insert it into the DOM.
Like this:
var someVariable = "-123456";
$mylabel.text( someVariable.replace('-', '') );
or a more verbose version:
var someVariable = "-123456";
someVariable = someVariable.replace('-', '');
$mylabel.text( someVariable );