Related Questions:
How can I change a custom field?
How can I update a custom field?
Procedure:
Click on Settings on the left-hand sidebar
Click Fields
Click Custom Fields
Select where you would like to edit a field by clicking on one of the sub-tabs (Contact, Account, Booking, Event, Group)
Hover over the Custom Field you would like to edit and click the Edit button
Make your changes
Click Apply
Editing Custom Fields will not remove or overwrite the data (in the case of HTML Custom Fields) in any existing Bookings.