I had this happen to me last year. It is an easy fix to change the seal if you mean the one which enters the gearbox at the foot lever.
It's easy to take the gear selector shaft out and I just leant the bike over slightly so the oil didn't run out then replaced the seal with a new one.
Just make sure that it is seated correctly and you don't knick one of the sides as you put it in there. I found using a socket with the correct diameter served as a good tool to seat the seal.