first of all take it to a bike shop if you cant change a tire!
dont know what type of bike you are changing tires on? you need to remove the tire first and secound deflate the tire if not all the way flat yet, third you need to get the tools to remove the tire, two flat blad scuw drivers will work but the best tolls are plastic blad tools you can bie at the bike shop or a sporting goods store used to remove the tire with out puncturing the tub, once you have the tolls you need to press on the bead close to the rim and wedge the toll along the bead and the rim, pull one edge of the bead over the edge of the rim and pull along the bead and along the rimtell the hole one side of the tire pops over the edge of the rim, then pull the tub out from under the one side of the tire and rim before you take the secound side of the tire, ( repair tire and install tire first then push the tub under the tire and rim and reinstall the bead back on the rim) inflat tub with small amounts of air if you still have problems with pinched tubs).