Hi I bougt a air conditioner with a 230v cord and I want to change el double outlet in my wall for a one outlet of 230 volt can I do it? I hope that I explain myself OK!

No, you most likely will have to run a new 230 V circuit.

Assuming that the wire is just going to that one outlet ( and we all know it will not be) and assuming the wire is heavy enough to carry the 230v. You could change the outlet to 230.

But no, you will need to run a new wire to that location and put in a breaker into your box for this

The 110 plug -in is probably on a circute with other plug-in,,
Like the man said you need to run a larger gauge wire from breaker box to the place you need 230 v,,

If you are talking about the two outlets, top and bottom, on a duplex receptacle, are more than likely on the same circuit and shared with others in the vicinity. If residential shared with your lighting as well. Wire size depends on amperage required ( you'll find FLA or full load amps on label or manual) and distance from your panel. If your not going more than 150 or 200 feet #14 which is most residential duplexes only carries 15 amps #12 most commercial 20 amps #10 30 amps #8 40 amps #6 50 amps american wire gague. Also to get 230v you'll need opposite phases from the panel. Working without knowledge can be dangerous request professional help