You are allowed to go in and out of Centeroo as much as you’d like - just remember to scan your wristband each time.
If you must leave the campgrounds for the “real world”, this is heavily discouraged, but allowed. First, there are general stores on site (now with MUCH lower prices) with things like beer, sunscreen, phone chargers, etc. and honestly going in and out is a big hassle given Manchester’s small roads and the festival traffic.
BUT if you have to leave, you are allowed. You will need to scan your wristband and car pass as you exit (this is VERY important so you will be allowed in again). Depending on the time and traffic you also may not be able to leave until inbound traffic has died down. This doesn’t usually happen until Friday evening, so do NOT plan to leave and come back on Wednesday or Thursday, or even Friday morning - again you should plan to avoid leaving if you can.