supplement, either with MSM or condroitin (or both)which is an over-the-counter preparation available thru vet suppliers or at WalMart in the human medicine section. It won't ease the pain, which might require pain medication also, but it will help loosen/lubricate those aging joints.
If it is a young dog that is afraid to go up stairs, start with 2-3 steps at most, either using front door steps or find some place where there are only a couple...lay treats on each step and use lots of praise. Often times its easier to teach a dog to go down 2-3 steps (no more, to start with) than to teach upward...same idea...using treats hand-held and lots of praise, get the dog to do a couple, successfully, then you can add the next week, a couple more, again using treats and praise. For treats, suggested using small diced cube of cheddar cheese (if dog isn't allergic)...its very tasty, very different=interesting.