You should first check refrigerant level, if its low top it up. Also check condenser, if its too dirty AC won't cool, happened with me on my Honda Civic, AC started cooling after cleaning the condenser with pressure washer.
If everything is clean (condenser, evaporator, blower, cabin filter) and refrigerant level is good, then check if compressor is working properly. I had Mercedes E-Class which started blowing hot air only, after checking everything it turned out to be a faulty compressor, replaced it with a new one and cooling was restored.
