To check your current default shell:
$ echo $SHELL
If it shows /bin/bash , then your current default shell is bash
Now to change the default shell (let us say to zsh), first you have to check if zsh (or any other which you need to make your default) is in your authorized shells list (/etc/shells).
The correct way for a user to change his or her shell is via the chsh command.
chsh stands for change shell, and does exactly what the name suggests.
You can supply the shell name with the -s argument:
$ chsh -s /bin/zsh <username>
where <username> is the user for which you want to apply the default shell.
OR, it can also work without showing the username:
$ chsh -s /bin/zsh