Reheating salmon is safe, and you get the best results if you do it in the oven, rather than in the microwave. Put the salmon on a greased sheet pan, and cook it at 375 F between five and seven minutes. It is also possible to drizzle some olive oil in a pan and warm up the salmon on your stove top.
Salmon does not keep forever, so when you take your leftovers out of the oven, give the contents the "smell" test. If the salmon smells unpleasant, you likely have waited too long. Play it safe, and eat something else.