Thankfully you don't have to do anything -- rest assured that Super Bit Machine will be pushing another build so that testers don't lose access to the beta because of TestFlights 60 day expiration.
In order to stay in the beta, however, it's important that you maintain your activity in game. Beta testers who do not maintain their presence in the game will eventually be pruned from the list of testers. This is done to make room for people who actively want to play the game, since Apple only provides 2,000 slots for TestFlight beta testers!
If you're not in the beta and want to play, hang tight -- Super Bit Machine is working on a wide release!