If your event has most of the buttons and icons that allow editing on the booking and detailing tab missing, the event is most likely finalized. Finalizing an event's financials locks it down to only allow basic event information and post-event information to be updated. Finalized events are marked as such in the status chip in the page header.
If you need to update something that has been locked down, you will first need to un-finalize the event. To un-finalize the event, you will need Detailing Edit permission.
To do so:
The event's information can now be updated.