Changing dihedral is very easy.  Just trial-fit the wings together at the desired angle and then use a piece of paper or cardboard to make a template of the wing joiner.  If you're building a kit, it's a very easy process.  If you're assembling an ARF, you'll have to make a center rib that's wedge-shaped from top to bottom to accomodate the gap you'll be making...either that or carefully cut out the root ribs of both wing halves and reinstall them after you've made and put in the new wing  joiner.

Finally, if you already have an assembled wing, you can lower the dihedral a bit by putting a saw cut in the center joint about 9/10 of the way from bottom to top, not cutting through the top of the wing.  Force epoxy glue into the gap and weight the wing to close the gap.  Lay 4"-6" wide 6-ounce fiberglass cloth across the joint from leading edge to trailing edge.  When the epoxy cures, you'll have a wing that's every bit as strong as it once was.  Of course, you'll have to remove the covering from the balsa where you glue the glass cloth.