Can I edit a W2 after it's completed?Two of my employees moved after being laid off for the 2018 season and I already filed their W2s in QuickBooks with the old address!

If they have not been submitted you can edit. Otherwise you have to issue corrected W2 and you cannot create W2c in QBO.

If all that is different is address you can wait until you have IRS confirmation of receipt and then go onto the IRS site to change the info. But you are not required to change address for a terminated employee beyond what it was when they left your employ. If it comes back as undelivered that i no way relieves the employee of their obligations  - technically you should only have to change the address if they give you a new W4