Lay the watch, back down & cover in position, on a firm flat surface, making sure the cover is in contact with the surface; if not, due to the shape of the watch or strap, pack between the cover & flat surface. Get hold of a suitably sized mechanic's socket, that clears the watch glass and bears on the surrounding watch body. Place a rag between the watch and socket to prevent scratching, put socket in position & press down hard. Mine went back on first time, good luck.
Handyman, May 2009