An applied adjustment is permanent - it has already been written to your stock and to Shopify's inventory history, so it cannot be edited or deleted. When one was wrong, Reverse is the way back: it writes a second document that moves the same units the other way, and leaves both on the record.
Reverse one
Open the applied adjustment and press Reverse. Stockroom opens a new draft adjustment with every quantity flipped the other way, at the same location and under the same reason.
Nothing changes in your stock or in Shopify until you read that draft and press Apply. Until then it is an ordinary draft: you can edit the lines, add a note, or delete it and walk away.
What the two documents show
Each one names the other at the top. The original reads Reversed by ADJ-1009 and the reversal reads Reverses ADJ-1003, so a month later the pair still explains itself.
That link is also what stops the same adjustment being reversed twice - the Reverse button is gone once a reversal exists for it. If you delete the reversal draft without applying it, the button comes back.
Good to know
A reversal is a normal adjustment, so it can be reversed too - undoing one by mistake is exactly the situation the button is for.
The draft prices and previews itself against today's stock and cost, not the day the original was applied. A reversal raised weeks later shows the change it will actually make now.
Only applied adjustments can be reversed. A draft is edited or deleted instead, and a cancelled one never touched your stock.
Adjustment history imported from Stocky cannot be reversed. Those movements were never sent to Shopify from Stockroom, so putting them back would move stock we never moved.
If every line on an adjustment changed stock by 0, there is nothing to put back and Stockroom says so.