[Evgeni]

Not disagreeing with previous advice, but I have found it quite difficult to screw the retainer and plug in with the tensioner fully released.

Your arrow is pointing to the tensioner. Fish it out - a spring will follow and there will be a piston as well which pushes against the chain guide.
Once you pull it out you can use a 3mm allen key to turn the cam sleeve which will set one of 3 positions. Fully closed, half open and fully extended. You'll get what I mean when you get it out.

The other bit in the photo is the retainer and plug - you will need to separate these. There are o rings on both that will need to be replaced plus a filter between both these parts that you can clean if dirty. Put the retainer in a pipe wrench to hold it and undo the plug with a 14mm or 9/16 ring spanner.

Set the tensioner to the fully closed position and use the retainer to screw it into the head. There are two tangs on the piston which need to be horizontal to lock into the chain guide. Then release the tensioner with your allen key to the fully extended position. Then you can put the plug back.