how do i change the username to realname in redirect text when we log in ?

"Thank you for logging in, x"

Under 2.5.5 or 2.5.6 if you look in the file kernel\user.php you will find a list of available selections from the user database entries.

uname is the key to use to get the user name.
name is the key to use to get the real name.

Not knowing what you are looking at I am assuming there is a call to get the user name something like:

$name = $xoopsuser->uname();

You would simply change this to:

$name = $xoopsuser->name();

I haven't messed with this stuff directly in over a year so my memory isn't 100% but it should be this simple if you already are already pulling in the user name.