The details of a voyage can be edited and maintained in the Voyages modules
When you have located the voyage click it and the details will show in a panel (so called "drawer") on the right hand side. Details can be maintained directly in the fields.
Change ballast port
Ability to change ballast port is only possible when the voyage status is estimate or unallocated
Change vessel
Ability to change vessel is only possible when the voyage status is estimate or unallocated, or when the drawer is not opened from FAS