changed apple id now problems

I have a MacBook Air. Unfortunately I didn't know that Apple ID should not be altered. Unfortunately, I changed my apple ID and now find I can't update apps with new apple ID. I keep receiving message saying the mac doesn't recognise my apple ID. I can't find a way to revert to old apple ID.. I've been to apple shop but they were no help and can't find anything in my 'How to do everything Mac' book.