The bottom o-rings are a bit more of a pain because, if your tube remains in the head, rather than in the cover, it can be a bit of a pain to extract. You have to be careful not to damage it as you pull it out.
Once out you can flick out the o-ring with a seal pick. Then give the area a blow out with compressed air and a tub with aerosol carby cleaner or the like before taking the plug out. Clean it first so you don't blow or wash crap into the cylinder through the plug 'ole.
I then lightly grease the new o-ring with rubber grease and install it in the groove, this can be very fiddly, be prepared to walk away and come back in five minutes! It will go in eventually. Wipe off excess grease and I give the bottom of the tube a dress-up with very fine wet and dry to de-burr it and get off any mank like baked on oil.
You can then, if you wish, (I only do this if I think the surface of the tube has been compromised.) put a very thin smear of Threebond 1211/Hondabond/Yamabond around the bottom of the tube before slipping it back into the head/o-ring. Replace the top one while you're at it but don't use Threebond, reinstall the cover, then the plug and cap. Cross fingers. Go for a ride.