Simply replace it with nothing:

var string = 'F0123456'; // just an example
string.replace(/^F0+/i, ''); '123456'

var mystring = "crt/r2002_2";
mystring = mystring.replace('/r','/');

will replace /r with / using String.prototype.replace.

Alternatively you could use regex with a global flag (as suggested by Erik Reppen & Sagar Gala, below) to replace all occurrences with

mystring = mystring.replace(/\/r/g, '/');

EDIT: Since everyone's having so much fun here and user1293504 doesn't seem to be coming back any time soon to answer clarifying questions, here's a method to remove the Nth character from a string:

String.prototype.removeCharAt = function (i) {
    var tmp = this.split(''); // convert to an array
    tmp.splice(i - 1 , 1); // remove 1 element from the array (adjusting for non-zero-indexed counts)
    return tmp.join(''); // reconstruct the string
}

console.log("crt/r2002_2".removeCharAt(4));

Since user1293504 used the normal count instead of a zero-indexed count, we've got to remove 1 from the index, if you wish to use this to replicate how charAt works do not subtract 1 from the index on the 3rd line and use tmp.splice(i, 1) instead.

JavaScript strings provide you with replace method which takes as a parameter a string of which the first instance is replaced or a RegEx, which if being global, replaces all instances.

Example:

var str = 'aba';
str.replace('a', ''); // results in 'ba'
str.replace(/a/g, ''); // results in 'b'

If you alert str - you will get back the same original string cause strings are immutable. You will need to assign it back to the string :

str = str.replace('a', '');