How do I open up a rexel stapler?

Here is the manual.

https://www.manuals.ca/rexel/optima-70/manual

and a link to Swingline for repair under warranty.

Swingline Optima 70 Desk Stapler, Reduced Effort, 70 ...

https://www.swingline.com > staplers > optima-70-desk-...
Shaped ergonomically with a soft handle, the Optima 70 requires less effort to use in comparison to traditional, mid-capacity desktop staplers · Jam Free ...
Rating: 1.7 · ‎6 reviews · ‎US$47.00 · ‎In stock

Sadly, the solution to this problem is very simply, and stated on the instructions for the stapler, which I ignored myself until I had bought my third one and continued to have the jamming problem. The solution is you HAVE to use the Optima staples they sell. Look for them in the silver metal box, and they are labeled Optima. Once I did that, all three of the staplers worked without a hitch, and I love them. Good luck!

If you imagine that the end that the staples come out of is the front, then the back is where you load the staples. Your picture shows a small black bit at the very back of the stapler. You have to remove this, usually by pushing it in and lifting it up. Once loosened, it should pull out from the back of the stapler. You should then be able to reload. Finishing is the reverse process.

Rapesco Part number is: S14082M3. I've used these without problem

apparently Rapesco has now changed to Tacwise and they are now sold as
Tacwise 0346 (Type 140)

When you lift up the top of the stapler (as though you are going to put staples in), does the pusher slide freely back and forth? It might be getting stuck at some point and not pushing the staples (either due to a worn pusher spring, or friction). First (after removing the staples) try spraying some light oil on the mechanism and lifting the top part of the stapler up and down a few times and observe how easily it moves back and forth. Also, with the top open try GENTLY sliding back and forth a little - if it slides fine with no sticking, try loading staples and using stapler. If it slides ok, but doesn't quickly return to it's "normal position" when pulled so as to put tension on the spring, rhen the spring may be worn and need replacing.

Rexel Odyssey heavy duty stapler: The pring that pushes the staples into position can come losse if the stapler iss dropped. It's difficult to work out how to replace it! However, there are only two "lugs" that will take the circular ends of the spring. One obviously fits to the sliding staple retainer. The other to the back end of the pressed-metal inner body. However, to work, the spring must be threaded from this point along the pressed metal parallel-sided inner body back through a slot and over a rounded bent piece (shiny from prvious friction) and back down to the sliding staple retainer. It's trickey to do, but that's where it has to go in order to provide forward tension when the Stapler is closed.

The same here

I have one equally old and my plastic bit broke off many years ago.
I just get used to using it without.
It sticks but it's such a good stapler I forgive it.

You'll have to unplug the machine, then remove the screws front (inside door or cabinet) and rear to remove the top cover. Insure that the safety cover arm which contacts the microswitch is not broken or the microswitch is out of adjustment.

Hope the following is clear if not write back and I'll try to clarify, if you look at the body of the stabler molded to the body there is a bump that is round on the botton and kinda notched on top. the round part of the spring fits around the bump. The spring has a straight side and bent side. the straight side rests against the a flat bump which is slight back and lower than the round bump. the bent side clips into the notch on the handle (clip this end in last) I used a plier but there is very little tension involved the spring just returns the handle to the at rest position,