I just wire 4 new outlets in my basement room. I use 14 wire and 15 amp outlet.When I went to hook on to the outlet that was there.I found out that the outlet was on a 20amp breaker.Can I just change the 20amp breaker to a 15amp breaker.thank Dave

Yes.

Kind of a shame to reduce the circuit to 15A due to the smaller wire, but you have no choice.

Why not run #12 and keep the 20A circuit?

Yes, you can downsize but you will have reduced capacity, about 600 watts. How hard would it be to go back with 12 AWG. What is the AWG of the wire you are trying to connect with, 12 or 14?

WOW, 3 of the same answers in 5 minutes! Way to go guys!

Check to see if you have any 20 amp receptacles, different than 15 amp, if you do then swap them out too.

Yes, you can, as long as you don't think you'll need more power in the future.