On a built model the only way out is to sand the bottom or cut it with a sharp knife, I would recomend the first!
Heating would risk too much damage to your finished kit!
When building a kit the best way I have found is to do it over an iron, heat it up place grease proof paper over the iron and press down to get the desired effect.
I like to use a spares box wheel to give the tyre sone rigidity without the danger of damaging the wheel that you want to put onto the model.
The only way to find how this best works for you is to try it with some spare tyres that you dont intend to use.
This works with all plastic and rubber tyres.
I guess I dont need to say this but dont try it with resin ones!!