How to use Interact with the corner of Robin's shop counter (left of her register) to begin upgrading cabins. You can also enter upgrade_cabin into the SMAPI console window to show the menu anytime.
You can upgrade any unowned cabin at the usual cost and wait time of 3 days. This mod doesn't touch cabins that have an owner. You'll find Robin working inside the cabin while it's being upgraded like usual.
You can remove the seed boxes from all unclaimed cabins by entering remove_seed_boxes into the SMAPI console window, and remove beds using remove_cabin_beds.