Plans change. Doing this properly takes a minute and saves a coordinator an hour.
Cancelling
Open My shifts, find the shift, and choose Cancel. The seat opens again immediately, and if anybody is waiting for it the first of them is moved in and told.
There is no penalty for cancelling and no approval to wait for. Do it as soon as you know — a seat handed back three weeks out fills itself, and one handed back at 5am does not.
Offering a shift to another volunteer
Instead of cancelling outright you can offer the shift. It goes on a board other volunteers can see, and the first person who takes it takes your place. Until somebody does, the shift is still yours: an offer nobody accepts is not a cancellation, and you are still expected.
Some organizations have a coordinator approve swaps before they take effect. If yours does, you will see the swap marked as waiting, and both of you are told when it is decided.
The morning of
If it is a few hours before the shift, cancel and tell somebody. The reminder email you received names the person to contact. Nothing in this software wakes anybody up, and a seat that opens at dawn is a seat a person has to fill by phone.
If you simply cannot make it and cannot reach anybody
Cancel the shift anyway. A record that says you were not there is more useful to everybody, including you, than a record that says you were.