How To Increase Username Digit Ability

in jcmc user can register 2 to 15 digit username and password 3 to 10 ... but i need a script by which i can increase the username abilty 3 to 20 and password 6 to 20 ... can any one help me about this, plz help me in details..

you need fix file registration.php in following string: 33, 36, 70, 77

---
strings 33, 36: maxlength="15" & maxlength="20"
&
string 70: elseif (mb_strlen($reg_nick) < 2 || mb_strlen($reg_nick) > 15
&
string 77: elseif (mb_strlen($reg_pass) < 3 || mb_strlen($reg_pass) > 10)