If a client is having problems receiving their password reset email, you can update the password directly from the clients profile.
First navigate to Clients > Client List
From there, select the client you want to add a password to
When in that client's profile, you will select the Security tab
From there, scroll down until you see the Password section and click on the Change Password button
You will then update their password and click on the Save Settings button below
That option is usually used as a last resort. Typically you would want to click on the "send a password reset email" link underneath the Change Password button.