If you push the bead down into the centre of the rim then you can normally get a bit more slack on the tyre. Then you have to try stretching the tyre round the rim without letting it slip. Which is a lot easier said than done. Should go on then, but some rim / tyre combos are a complete pig. It is normally possible to take clinchers off in the same way. push the bead into the centre, stretch it round the rim then pull it off like its a really really stubbornly glued on tub.
If you push the bead down into the centre of the rim then you can normally get a bit more slack on the tyre. Then you have to try stretching the tyre round the rim without letting it slip. Which is a lot easier said than done. Should go on then, but some rim / tyre combos are a complete pig. It is normally possible to take clinchers off in the same way. push the bead into the centre, stretch it round the rim then pull it off like its a really really stubbornly glued on tub.