It's fairly simple.
Usually professional framers put a brown, Kraft type paper on the back of the frame. It's cut to size, and glued around the edges. You can either
(1) cut the paper around the inner edge of the frame, and remove it, or (2) use something like brake cleaner, spray it on a paper towel, and wipe the edge of the frame. This will release the glued paper from the frame, then you can reapply it after you put the new poster in.
With wood frames, framers usually use rigid points to secure the foamcore to the frame. Use something like a flat-head screwdriver to press against the side of the point, and alternate sides, to wiggle it out of the frame. Then you can remove the foamcore and the poster.
After you put the new poster in, and put the foamcore back, you need a point driver to put new points in. You can use rigid points, that are a little stronger, but can only be used once, or you can use flexible points, which are not as strong as they can bend fairly easily with something like a flat-head screwdriver, and can be used several times before they eventually become too weak and/or break.
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TL;DR: Yes. Remove the paper on the back of the frame, and you can figure it out. If they're wood frames, you'll need a point driver. I have this model and it works great.
http://www.amazon.com/Logan-Pro-Framing-Dual-Point-Driver/dp/B000WZS9Q0