There is some stuff about adjusting nexus on the shimano europe website (FAQs), but apparently, how to remove a nexus wheel in not a FAQ.
Your dealer should have supplied an instruction sheet.
I find that track nuts, on solid axles respond best to a ring-style wrench (uk spanner), rather than an open crescent style. I also find that a bit of gentle persuasion on the wrench using my foot works well.
You can get inner tubes which are cut and sealed (ie not circular), which makes replacement a lot easier.