For a single contact - You can change the user's password via their contact profile. After you manually set a new password, there's a checkbox "Request change of password on next log in". It's worthwhile checking that there are no duplicate contacts using the same email with a password, because that may confuse things.
If you need to handle this for a lot of contacts in bulk - there's a tool "Update Profile Email", which is available via Dashboard > Customers > Contacts, from the menu at the top of the page. This will create a drafted email for you to bulk send out (with auto-login links - so don't forward the e-mail).