feed the strap through then use a knife to push through, actually it`s quite easy I`ve got 2 x straps on mine and what a difference it makes for comfort ...Bluey
Brute force and flex to get them out (you could also slide something down the inside of the dovetail to make it easier if your not too good with the brute force thingy)
Then as Mr Bears says butter kinfe or cake slide down the inside of the dovetail to get a new strap in (if its a worn pair of frames should be able to twist a bit and shove a corner of the dovetail through no problem)