Yes, you just need to click Edit Transaction and make your changes.
When tickets and/or products are changed/added to the basket, this change will apply immediately.
When a Delivery Method or Customer Details are changed, you just need to click Continue and then that change will be saved.
You can then either leave the page or click Finish Editing Transaction on the final page.
NOTE - Reservations will remain in Reservation status (i.e. instead of 'In Basket) even if they're being edited. This means that Reservations will never expire when being edited.
If you would like to remove the Reservation, then click Clear Basket when editing the transaction.