Can I undo a delivery I checked in?

If you've checked in a delivery and some of the detail was wrong, for example maybe there was some adjustments that should have been made to the items checked in, there is no way to re-open the existing closed delivery record (held in Ordering > Deliveries > Completed).

However, depending on your required outcome you could make adjustments by adding in additional Delivery or Return records to affect the stock qty deposited, or if it was just price adjustments that you needed to make you can adjust these in the Invoice record in the Accounting section (Accounting > Payments > New Purchases > Actions > Edit).