thx to eruo614 c-hing, I understand the question. here's my 0.0039 cent's worth
1. CAREFULLY cut along the edge of the tire and rim to remove. I have tried this and results have been good/bad depending on how patient I was at the time.
2. I've heard people using acetone (nail polish remover). Put the wheel in a metal tin. Then put in a wad of cotton wool/paper soaked with acetone. Close lid and leave over night. Remove tire from rim by hand. Note I have never tired this and apparently some plastics and tire do not react well to acetone. i.e. you might be left with an unusable tire + rim at the finish.