If you use at least two tyre levers, three would be easier and start from the valve and "walk" them round, i.e. put the first lever in to break the seal then the second lever to elongate the break then remove the first lever to continue breaking the seal and by doing that until you get round to the valve the tyre should be free from the rim. Undo the nut that is holding the tube in then remove the tube. When inserting the new/repaired tube insert the tubes valve into hole, screw the nut on and work it carefully around inside the rim of the wheel, when tube is inside the rim slightly inflate the tube and smear plenty of lubricant around the tyres edge, neat washing up liquid is ideal then use the tyre levers to prise tyre back into the rim being careful not to pinch the inner tube.
I presume you know how to patch up a tube. So easy to do so if you don't I will go through step by step.