Go to the login page and click on Forgot your password?

This will prompt them to enter their email associated with the account. After they have done this, they can click on the Request New Password. Once this is done, they will receive an email with a reset password link. This may go into their spam or junk folder.

If your customer's have trouble resetting their password, you can do this through the back end.

Go to the Customers tab on the left hand side and locate the customer. Click into the details section by clicking on their name again and scroll down. Here you will be able to see their email address and below this will be an option to enter a custom password to replace their existing password. This can also be generated using the Generate? button. You can send these login details by clicking the tickbox. Make sure you press Save if you make any changes.