You'll need an editor, like nano, vi, gedit, kwrite or whatever. I like nano myself, very easy to use, but don't know if you have installed it. Vi is installed on most systems. You can do it from the commandline by:
to edit your file. In your case, open the shell, cd to your home directory (if you aren't there already), type "vi .bash_profile".
Or if you don't like working from the shell, open gedit or kwrite. Open .bash_profile from gedit or kwrite, and edit...
By the way, the file .bash_profile could be named .bash_rc too.