I'm assuming you're referring to the seals on top of the plastic cover. The ones on the side where the injector lines go in are easy to replace.
Here's a video from India and it looks easy ... especially when the new seals are just pushed in with his thumbs!
The socket that holds the seal is conventional compared to a different design on my 2011 Thai camshaft cover.
Last year a few days before a remote expedition I noticed numbers 3 & 4 were leaking.
After removing the cover it was almost impossible to get them out without cracking the plastic. I had to destroy each seal by prising out with a screwdriver. I was going ok but the last one put a crack in the plastic that forms part of the breather chamber. Then I phoned the dealer and was informed the seals were not sold as a separate part they only sold the cover with new seals for $AUD228. I didn't have the time to investigate seals from elsewhere so beat the dealer down to $178 and bought a new cover.
If I was doing the job again I'd try a small dremel type wheel to cut the seal ... hopefully without cutting into the plastic but it would be difficult. Once cut it should be relatively easy to prise them it out.
Looking at the cover I feel they're not designed to replace the seals as a maintenance item. There seems to be an additional flange the seal slips under to lock it in. They certainly won't push in by hand like you see in the above video. They'd have to be driven in without breaking the plastic.
With the cost of dealer labour $AUD150+ /hr I'd suggest it was cheaper for them to remove and install a new camshaft cover compared to the time involved in getting old seals out and new ones in without damage. My local Nissan dealer confirmed that was the case as they were always breaking covers.
If money is not a big problem I'd buy a new cover and make it easy for yourself.
When I installed the cover I smeared some rubber grease around the seals and it's all been clean and oil tight ever since.
Here's another video of a Nissan plastic camshaft cover with the seal flange that's similar to our Thai Navaras. The guy explains why the whole cover has to be replaced.