Hello. I recently bought S3 and many vehicles have cutters around 8000-9000 rpm. However, the maximum power speed of the vehicle is 5750, but it reaches very high revs. Any chance of adjusting this myself?

Hey.
In short - you can't directly edit them.

You can make and submit a derivative version if allowed by the author. If not - you can always ask the author for permission.
It's written in the mod description page under license type:
Derivatives are allowed: YES

or
Offline only:
You can locally edit the .veh file and use it offline only.
Example: go to LFS\mods\vehicles\BZ30 ROADSTER~034\ and copy BZ30 ROADSTER~034.veh into LFS_EDITOR\data\veh\ -> edit the .veh inside the Editor and save it -> copy that modified veh file and paste it in the LFS\mods\vehicles\BZ30 ROADSTER~034 folder replacing the original one. Always make a backup of the original file or just delete it and it will download it automatically if you want to use the mod online.