Businesswise I would like to give multiple options to user, as the app would become unusable if user does not have the password.
Option 1: Provide answer to a security question (which user will need to register beforehand) and you can reset the password. For this, the solution will be coded within the app.
If Option 1 fails (user does not remember the answer)
Option 2: Ask for the email address/ login id of user and send him password reset details. Now this will need to have a webservice (to send the email address) and a database (where you will validate the email address against stored user record). This obviously means you have to create a small server side web service.
That ofcourse is ideal scenario. If you do not have bandwidth/ resources for option 2, you will have to skip that.