Not a big deal to replace the seal. While you're at it, you might want to install a new bearing set and front seal too. Not very expensive and if the current ones are originals a replacement on both sides should be considered. Bearings will last forever if properly greased and tightened, but you'll have to pull the wheel and existing set so its a good time to swap them out. When you get the hang of it on the problem side, its a good time to use that experience and go ahead and do the other side. The ugliest part of the process is repacking the new bearings. Not hard, just messy. Most prefer to do it by hand rather than use a packer.
You can also take it to a service center and have it done professionally as the cost depending where you live should be less than $100.