this job is best done with 4 hands. call up a friend and have him help.
first, flip the sight leaf up to about a 60° from horizontal angle.
under the sight leaf is a flat leaf spring. using a screwdriver, depress the spring in front of the pivot point of the sight leaf.
this will require a lot of pressure!!!
one slip and injury, or at least lots of swearing, is possible. puncture wounds suck!
let your friend hold the rifle securely and press down on that spring.
while he takes the pressure off the sight, wiggle it on a side-to-side angle tp get one of the pivot posts into the large part of the keyhole slot in the sight base.
now, careful twisting will worm the sight leaf out of the base.
installation is just the reverse of this.
another option is to modify a c-clamp...thinning down the clamp screw and blocking the receiever bottom with a thin plate of steel to clamp against.
i have also heard of guys that used a wood golf tee, driven by a c-clamp to depress the leaf spring.