How do I undo a disassembly?

Only by a reversal, with its reason.

Steps

Open the disassembly and, under Reverse this disassembly, give the reason and click Reverse. It needs Production - Approve (or the owner).
The components go back out of stock at what they cost today, the finished goods come back at the cost they left at, and a reversal journal books exactly those movements - if a component's cost changed since (e.g. its average moved), the difference goes to stock adjustments, so stock and the ledger stay equal. The reversal and its reason are in the audit trail.

If the recovered components have been used or sold since, it can no longer be reversed - there is nothing to take back out.
A posted disassembly is never edited or deleted.