to ~/.xprofile (X11), ~/.bash_profile (bash shell), or ~/.profile (under certain case it won't be read if .bash_profile presents). It will be a hidden file, since the file name starts with dot. You may use any editor, like gedit, kwrite, vim, emacs to edit it.
to ~/.xprofile (It will be a hidden file, since the file name starts with dot), open your favorite editor, gedit, kwrite, vim, emacs to edit it).