AFAIK, there is no way to change things once planning is completed. Reservation is done as part of planning process.
The only way to clean it is goto Execution>Reservation > Delete or clean reservation. But unfortunately, for a mass deletion its a pain, as there is no way to select multiple and delete them.
Also in some cases, cleaning the reservation may not help as the reserved object would have already been created on the traget( 7mode or cDOT)