The simplest solution is to run chsh. Example:

chsh -s /path/to/shell/binary username

The command above will not work (chsh: /path/to/shell/binary: non-standard shell) if the shell You are trying to set is NOT listed in /etc/shells There are other ways of setting the default shell, more can be found here If you want to check which shell are You using type:

ps -p $$