If you are wanting to do this on a per-member basis, you can find their account in the ACP.
Click on "Edit Password".
You can either enter a password for the member directly (which will not be emailed to them) OR you can click the link to force reset the password.
If the user is currently logged in, they'll be logged out. They'll get an email prompting them to reset their password. If they follow the link, they'll be able to set a new password.
It is important the user's email address is still valid, etc for this step to work obviously. 🙂