How can I remove a 240V heater to make 110V outlet
There are 3 components to this circuit:
Breaker
You currently have a 20A double-pole breaker. That gives you 240V between the hot wires, as used by the heaters. Unless you go for an MWBC, you should replace that with 2 separate 20A breakers, and if you do that then one would be for the circuit you are using and the other would be available for a totally separate circuit with its own hot &amp; neutral wires. Note that I said 20A, not 15A. Note that you can get 2 120V circuits here, though if you really only need one then you can leave the 2nd space empty.  If you just put in a single 20A breaker then you should put in an appropriate blank cover plate in place of the empty space.
Wire
Note: Ignoring ground for this discussion. Ground should be a green or bare wire run along with the other wires and connected to the panel ground and to any appropriate device ground connections (metal case, ground screw, ground pigtail). You may also have ground connected by using metal boxes + metal conduit. Ground always needs to be connected one way or another.
You currently have red &amp; black wires. That should mean one of two things:

Individual wires run in conduit, and just two of them - black &amp; red. In this case, you should replace one of the wires with a white wire for neutral. Then connect the original wire (black or red) to the breaker, the white to the neutral bar in the panel, and wire up the receptacle on the other end.

12/3 cable which should have black, red and white (plus ground, of course). In this case, you will use the black or red (not both) for hot, plus white for neutral.


Receptacles
That's receptacles, plural. The good news is that an ordinary 15A dual receptacle can be placed on a 20A circuit. That way you can make use of the full 20A for loads. This is an exception to the &quot;receptacles must match breakers&quot; rule. If for some reason you only want 1 receptacle, then it must be 20A for a 20A breaker or 15A for a 15A breaker. But most people want to have 2 receptacles anyway. The only reason I know of to not want to use a 20A circuit is spending extra on the wire. But in this case you should either have a suitable 20A (12 AWG) cable or at least have 1 12 AWG wire (having to add a white neutral), so the cost is minimal to go up to 20A.
Note that since this is not a simple repair, rules for AFCI and/or GFCI may kick in, depending on your local code and the location of the receptacles.