It is only possible to change the pickup time of a taxi. In all other cases, the booking must be cancelled and re-entered. Please see https://help.i-gtm.com/cancelling-a-booking/ for help.
Why?
I-GTM makes extensive use of external data through API’s connected to, among others, flight databases, train timetables and address look-ups. The majority of bookings taken by the system are reliant on this 3rd party data for their integrity, so if edits were made, the system would have to reinitiate those external calls and attempt to recompile the booking object – an extremely complex task. For this reason it’s easier to simply cancel and rebook.