If one of your users has forgotten their password and cannot reset it themselves, they will probably come to you for assistance. Thankfully, resetting a user's password is a quick and painless process.
Step 1: From your Admin console, select Users.
Step 2: Hover over the name of the user you'd like to reset the password for. A box will appear with the user's details, and a Reset Password button will appear.
Step 3: From here, you can make a new password for your user, and even request that they change it on their next login, for extra security.
Step 4: Once their password is reset, notify the user so they can log back in.
If you would like more information, Google's video on the subject goes into more detail about how to manage your user's passwords.