Junking the tire, cut it off.
Keeping the tire; your basically carrying out a removal/installation like normal, apply pressure to the opposite side, apply lube into the rim as deep as you can on the bead, using a strong bar, covered in plastic/cloth or whatever, tuck it under the tire and over the lip, whilst keeping pressure on the tire opposite, also apply pressure going around to the bit coming off, it will pop back on. having the tire held down will help allot.
Just gotta get the bead thats seated as far into the rim dip as you can or, remove, flip wheel and re apply.