Changing a whole section of a cycle route

Is there a way to change whole sections of a cycle route?

Both cycle route 1 and 3 in Norway don’t correspond to the one on the official site of the norwegian road authority (vegvesen) around Ålesund. Nasjonal sykkelrute 1: Kystruta (route 1)
Nasjonal sykkelrute 3: Fjord og fjell (route 3).
Route 3 ends at Ålesund according to vegvesen and route 1 doesn’t go around Tennfjorden anymore, but goes to Ålesund and then across the fjord with a ferry.

Maybe I’m not using the right terms in the search box, if so please direct me to the answer(s) if that is the case.

Have you verified on the ground that the signs represent this as well? Over here, a lot of times, the official GPX routes are not 100% correct and don’t match the cycle route signs on the street. I wouldn’t change routes, especially remove parts, without double-checking that the signs aren’t there any more.

But to answer the question: depends on the editor. In JOSM, you just select the ways you want to remove, edit the relation and then click on the trash-icon. Editing routes is quote tedious nevertheless, and easy to get wrong.