Dont know the winch, but if its a circlip, get a pair of circlip pliers. Two tyes. One for inside and one for outside clips. Less than a fiver and a lot easier than struggling with a screw driver.
The manual will make it plain. The "circlip" is a flat section spring which goes around twice, so by the time you have uncoiled it and it is ready to release, there is no longer a spring force in it - but take care not to drop it over the side. Then the drum just lifts off, again be ready to catch the bearings if they decide to fall out of the drum! I had one where the bearings came off with the drum and the other left them on the shaft.
All told, it's not a difficult task, but choose a nice calm day for routine maintenance.
Once the circlip is off, place it in a container so that it wont go overboard. When you carefully ease te body of the winch drum upwards, be careful of the pawls and their springs. take them out and similarly place them in the container. nothing worse than loosing these bits overboard.
BTW only use 3-in-1 oil on the pawls and their springs.
Lewmar have their own grease which should be used sparingly, , but I have preferred to use Quicksilver general grease with teflon, as I think it lasts better.
Take a smallish cardboard box and cut a round hole in the bottom to fit over the winch. Tape it down at the edges. That way you shouldn't lose any bits over the side.