How do I edit the member's level expires date?

I have few questions which all relate.

There are subscriptions and membership levels.

A person (user) should not have more than one membership level, right?

What about the membership subscription being infinite and set for a year?

Is there a dashboard way to edit the level expires date right now while I figure this all out? I don't want people being billed if they shouldn't be being billed.

Thanks!