Below are the cases you might need to adjust the rear derailleur:
Problem 1: The chain keeps falling off
Please note, if your chain keeps coming off of the rear derailleur, the possible reason is the inaccurate derailleur adjustment. Turn and tighten the barrel adjuster to decrease the tension.
Problem 2: Auto shifting or getting stuck
You may encounter a problem that your rear derailleur was prone to shift on its own even without using the thumb-shifter. This is mostly due to the improper cable tension. First, adjust the "cable adjustment barrel" on the rear derailleur. Another cause could be the bent hanger since it is very soft. You could easily set it right in such case, making sure the derailleur cage is exactly parallel to the cogs and the hanger is not visibly bent.
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