It looks like you should be able to undo the pivot bolt nut, tap out the bolt, and slide out the blade. The hardest part will be undoing that nut as you’ve only got a flathead slot for purchase on the other side and the nut’s likely rusted in place along with the end of the bolt appearing to be peened over a bit (either deliberately or through being banged against stuff over the years).

The easiest way may be to simply sharpen the blade in place, I would expect there to be just enough room to slide a file in to sharpen the blade as you move it inwards.

If you do have to remove the blade put some penetrating fluid on the bolt threads and let it sit overnight before trying to undo it (ATF and acetone mixed 50:50 works well or buy one of the commercial sprays).

Once you’ve got to the point of reassembly how tight you want to go will depend on whether you’ve got something fixed at the pivot point acting as a spacer. You might find the bolt is stepped with the threaded end being slightly smaller, or potentially a bush around the bolt. In this case you can tighten the nut firmly as the bolt or spacer will maintain the distance between the two plates.

If there is nothing to maintain the spacing between the side plates you can’t tighten the nut right up as it’ll push the plates into the blade and stop it moving. In this case tighten the nut gently until the blade is moving freely with little free play side to side, and you will want to use thread locker on the threads or peen the end threads over to prevent the nut coming undone again otherwise it’ll likely work its way off over time.