Hi Markus,
no problem, all the old documents are still available at Mark’s issuu page,
just scroll down and download the manuals, exploded views are in there 
https://issuu.com/mark77a
Edit:
Direct links;
Strida 1 manual
Strida 3 manual
I think you have to remove the rear wheel otherwise it is not possible to change the belt.
Best wishes,
Chris