can I make an AC colder?

If the air conditioner is old, it may have lost coolant, and will need to be recharged (best to use a professional unless you are very handy.)

Most likely though is that you are trying to cool beyond your A/C’s capacity. If your room size is too large (room size will be on the box it came in), if you have a drafty room or stairs that go down or open doorways (all of which allow the cool air to escape), or a clogged filter (which would not allow air to enter the A/C - no air in means no air out).

If you haven’t the resources to upgrade, you might be able to arrange to cool a smaller area by putting up a barrier to subdivide the room, a curtain or even furniture (it wouldn’t have to be floor to ceiling because cool air sinks,so you would just have to contain it.) And finally, if you have the option to move the A/C to a smaller area (a den instead of the living room, for example) it would help out a lot. Hope at least some of this helps, or at least gives you some ideas. Good Luck!!