Make sure the bead of the tyre is squeezed down into the channel in the rim.
The flimsier the tyre the easier it is to do this/the more likely the bead is to stay there once you put it there. Marathons are pretty heavy duty so it takes a bit of wrangling but once you get the bead in the channel they'll go on and off as easily as any other tyre will on that rim.
Make sure the bead of the tyre is squeezed down into the channel in the rim.
The flimsier the tyre the easier it is to do this/the more likely the bead is to stay there once you put it there. Marathons are pretty heavy duty so it takes a bit of wrangling but once you get the bead in the channel they'll go on and off as easily as any other tyre will on that rim.