it isnt always that simple, some tyres are made to sit really deep in the rim walls so you can run them at lower pressures, if you got a tyre lever set its pretty simple, push the bead into the middle of the rim, pull the whole tyre over to one side on the circumfrence of the wheel, put a tyre lever in, lever it uphook onto a spoke, same again next to it, then put a third in (all sort of close to each other) then take the middle on out n put it somewhere at the end, repeat and eventually the tyre will just pretty much remove itself