I accidentally created a shipment with wrong quantities and put the shipment in the done state. I saw in #11663 that it is possible to cancel the shipment, but I don’t know how since there is no Button in the UI.
What is the best way to do it?
The issue is not about that.
Indeed you must make a return shipment for the extra quantities.
If you invoiced also too much, you should link the return stock move to the credit note line so stock accounting and price computation will be correct.