Without seeing all sides of the shaft end I can't tell what type of locking the coiver uses, as there are probably a half dozen systems used out there. But usualy there's a lock tab, sometimes 3, that gets pushed with a screwdriver until disengagement from their locked position. With the 3-tab locking they all need to be released simultaneously to slip the cover off. Yours looks like a single lock tab, so pry that tab where it wants to go and try twisting off the cover with the tab in its new position. These kind have the lock tab sprung into place preventing the slipring inside from pulling out until it's rotated, which pushing the tab in allows. The slip ring will need to be pinned in place from inside while rotating the cover, maybe a screwdriver pushed against it from inside of the end will pin it in place so the cover can loosen.
(edits for spelling, grammer and clarity)