What is the best way to rollback a completed workflow?

You can:

a) set the active/current version via the Service Manager so all new instances will use that version (K2 Service Manager > navigate to the relevent Process node, click the 'Versions'.

b) re-export the desired process design  (if you still have a copy of the 'good' version)

If using web forms, were those altered as part of the changes made betwen your last 'good' design and the 'failed' process design? If so, then you'll have to roll those changes back too.

Are there proocess instances started from the "failed" process version? If so, those will continue to run on the 'defective' process design (or delete them via Service Manager).

HTH

Ack. Sorry. No, "completed" workflows can not be deleted.

but, you can move them away from the 'live' workspace into an Archive system so users won't see them. Ddownload/install the K2 Archiving utility.

The catch there is that it is date-based archiving of completed processes only...so if a 'good' process completed within the dates of two 'bad' processes, it will be moved off-line as well.