When I change the gear from 3 to 2 the gear shifter does not properly change the gear. It will just move to gear 1 then it pops off.
I put the chain back but it still had the same problem.. I don't know what to do . Please help me.
Not sure if you are having problems with the front derailleur or rear, but the answer is really the same.
Derailleurs have 3 adjustments:
High and low limits stop the derailleur from pushing the chain off the inner and outermost sprockets. Indexing lines the derailleur up with the individual sprockets. It sounds like you have both a limit and indexing problem.
If you are unfamiliar with bicycle maintenance the easiest option is to have a local bicycle repair shop do the adjustment. It's a straightforward process and inexpensive.
If you want to attempt adjustment yourself, there are many good 'how-to' articles and videos available that walk you through the process. My favorites are those produced by the Park Tool Company.
Rear derailleur adjustment is relatively straightforward, front derailleur adjustment is more complicated chain enters the derailleur at different angles depending on the rear gear selected.