It can be done by the owner. From what I've heard it is a messy, heavy PIA. Instead, my dealer replaced three on a used L48 that I bought.
The best bet is to take it off and haul it to a dealer or place that will replace the seals for you. It may take specialized tools and techniques. They can also discover is the ram is bent which would cause the seals to leak.
I don't think the Kubota cylinders are very hard to build and you don't need special tools either but the correct knowledge is needed for sure. I would have to agree with GWD and take them off myself and have a HYD shop build them for you. Unless they really stick it to you the labor for a simple rebuild will be fairly cheep and you should get a warranty with them which could be a good thing in the long run. I just had a hoist cylinder built for a customer of mine (Bobcat trackhoe) and the labor was about $40. I thought that was a pretty good deal and it's guaranteed for 3 yrs.