if you have to you can use large screwdrivers to assist in taking off the tire but you must be very carefull not to damage the rim plus not to pierce the tube, i suggest you firstly pore some hot soapy water onto the tire make sure all pressure is out then very carefully insert one driver but not deep then pull this back abit then insert the other driver whilst removing the first and so on until you can use your fingers to remove the remaining tire. the idea of this is to not damage both the rim and tube so be carefull and all will be fine. don,t forget the soapy water when you fit your new tire to the rim.