The grill should come right off with 4 screws. They may be in the back of the amp if you don't see them on the front.
There are about 1,000 staples holding the original one on. It takes a little time to get them all.
I use a tiny "Standard," or flat-head screwdriver that has teeth on the blade to loosen them. Then, I use a set of side cutters that are off-set at an angle. This makes prying very easy.
After you remove the original one, use it as a pattern for your new one.
Cut the new one out and then save the old one, just in case.
George, "GTS" says that it is easier if you soak the new grill. I think that is correct. I've done a few without soaking them and it is hard to line up and keep tight.
So, anyway, soak the new one for a while, until it is extremely ply-able.
Then, stretch it out over the frame.
I usually start on one of the longer sides, making sure that everything is as straight as it can be. I put about 1/2 of the staples in, or 1 every inch or so.
Then, I do the opposite side and stretch it as tightly as possible. Staple this side.
Then, do the other 2 sides.
Finally, after making sure that it looks the way I want it to, I staple the hell out of it.
Let it dry and you're good to go. Just saved a bunch of money and have the satisfaction of doing it yourself.
I also use 3/8" staples on the thicker grill cloth. Not sure what that comes out to in metric, maybe about 1 centimeter.