What is the the difference between the new MTB Altus, Alvio, Acera? What is the difference between these derailleurs. I want to change my derailleur and don’t know which one is better.
In increasing quality (and expense) is Altus, Acera Alivio.

In real world conditions, there is not a lot of difference between each set. Experienced riders will probably notice that Alivio is better than Altus, but other factors such as shifters, cables and drive train wear make a larger difference

General advice I give to  people is buy the right component - i.e. that matches the number of speeds and cage length - that's on the largest discount but still in your budget.