If a rental contract for an apartment is in effect; can the landlord force an amendment to the contract without mutual consent of all parties?

A written contract can only be amended in writing.

I would say it's impossible to force someone to sign something they don't want to sign (short of holding a pistol to their head).

If a rental contract for an apartment is in effect; can the landlord force an amendment to the contract without mutual consent of all parties?

I have seen leases which refer to a separate set of rules. The lease will recite that the LL may amend the rules from time-to-time and that such amendments are effective upon advance written notice to the tenant. Are you talking about something like that?