Can I replace a 26"/2,0 rim with a 26"/1.75 rim?
2.0 is the diameter of the tyre/tube in inches.  So a 1.75" tyre will be smaller and slimmer, and probably a little faster.

26" is the diameter of the whole rim, but check the tyre for its ISO number to be sure.  Its likely to be a ISO-559 size.

answer: yes that rim will probably fit fine.  You'll need to check and adjust rim brakes, or if its disc brakes then that needs to be right too.

Front wheels tend to survive better than back wheels.  If you're after a replacement wheel search out a local bike cooperative.  Our has dozens of spare front wheels and a lack of good rears.