var str = 'asd-0.testing';
var regex = /(asd-)\d(\.\w+)/;
str = str.replace(regex, "$11$2");
console.log(str);
Or if you're sure there won't be any other digits in the string:
var str = 'asd-0.testing';
var regex = /\d/;
str = str.replace(regex, "1");
console.log(str);