Once the terminal is open, write the following:
If you are asked wether you want to continue or connect, type in y for yes and hit return.
Now you can run a command named passwd to change your password. First type the following into the terminal:
You will be asked to provide your current password. Next you will be asked to type in your new password, and to retype it. Just follow the directions.
If everything works as expected, you will end up with the following confirmation message in the terminal:
passwd: all authentication tokens updated successfully
and hit return, and you are done. Please remember to update your password in your mail client.