1) Any manual for an AKM will teach you whatever you need to know about how your rifle works... Google should be able find that for you
2) You are stuck with the side folder, your current receiver is cut and drilled for a side folder rear trunnion. No feasible way to change that for an under folder trunnion.
2) You are stuck with the side folder, your current receiver is cut and drilled for a side folder rear trunnion. No feasible way to change that for an under folder trunnion.