Not straight forward and one of the big probs with the Trienkel, front suspension.
The spring exits the top of the tube. There should be three fasteners holding the triangular plate on. The damper shaft attaches to the middle in a fluster of rubbers and washers. Four fastners suggests a prior visit and issues.
To remove the damper requires that you remove the rocking hub carrier assembly from the ali King Pin/Steering casting. It removes from below after the collets remove loosening it from the spring. This is not to difficult IF the silent bloc/metalastic bushings are not rusted to the stub axles. If they are, they can total buggers to shift ending in having to be destroyed. Indeed it is probably easier to fight such a job on the bench, which means removing the kingpin. Itself a potential battle royal. If you want the spring out you need to do that anyway, or drop the whole front axle, 12 bolts and a union from memory.
Once apart you might find that you have an Armstrong damper. Oh dear! Boge dampers seem to have bits to repair them. Armstrong, nil point! Marcus is correct on the damper.
In pulling all this apart you might find more than you bargained for as there are many and varied things that can wear or be put together incorrectly. Big amongst this are worn suspension rocker bearings. The club are finally out of new stock and looking for replacements. The needle roller is standard but to have two with an intergral space is not. So there is hope as a bit of engineering should solve the missing spacer. If your car has already destroyed its suspension stub axle you maybe on bronze bushings. The stub axle itself was nitride toughened so once the hard surface has gone it wears fast. Its a press fit into the meaty rocker at a large pressure. As I understand it a hard steel pin would not grip, might shear, in this design of construction. So the less hard and durable option was used. The outer rocker bearing inserts can wear but survive better. So if this all looks horrible you need to think about if you can put it back together without worrying about it or will you want to fix it at the same time. Hopefully you will not have to.
As above, if restored correctly the axle will remove relatively easy. There might be a plan to rebuild this subassembly over the winter if it is not very good. In the days when these cars were in regular use the Trieklenaut always had a spare rebuilt axle ready. Any major issue was resolved with an axle change and then repair of the removed one under minimal stress.
Lastly. On no account hit the ali suspension casting with a hammer. They are not that strong, the metal is over 50 years old and crystallized a bit, and will shear off the steering arms and crack around the damper tube insert joint due to expanding corrosion, in the main (the steering rack brackets do the same and this is a serious failure -have you looked?). The RHD special casting with extra arm is practically extinct and at least 3 cars are sat without. I have several spares but the owners of those cars seem convinced they will find a replacement for a few quid. Good luck with that. LHD are far less of a problem, but the supply will run out. Besides it is needless to damage your existing one even if it can be most uncooperative.