All you need to do is go to your OMSI Directory, then Maps > BOTW_LH 2.7 and then at the top there will be a .cfg that is called AIlists. Open that with either Notepad or Notepad++. Then you will see the AI cars directories, below that is the buses. Where the bus directory is just change that to the bus that you want. When changing the bus, you must enter the directory as follows Vehicles/[Folder of bus]/[Name of bus].bus It is important to define that it is a .bus file or it will not recognise it.
Alternatively, just install my AI List mod that replaces the B7TL with the C400R.