DRAG DROP
You administer three Microsoft SQL Server 2008 R2 instances.
Database mirroring is configured in High-Safety mode with Automatic Failover between the
following three servers:
SQL1 is the Principal server.
SQL2 is the mirror server.
SQL3 is the witness server.
You need to upgrade SQL1 and SQL2 to SQL Server 2012. You need to ensure that
downtime is minimized during the upgrade.
Which six actions should you perform in sequence? (To answer, move the appropriate
actions from the list of actions to the answer area and arrange them in the correct order.)
Answer: 
Explanation:
http://msdn.microsoft.com/en-us/library/bb677181.aspx
“Add SQL3 back to database mirroring solution” should be the 4th step (and not the last one) according to: https://msdn.microsoft.com/en-us/library/bb677181.aspx
I think the order in question is correct.
If you read the step 5 and 6 in the link you provided, it says,
“Automatic failover will not work until the witness has been upgraded and added back into the mirroring session.” And then the last step 6 is,
“Upgrade any remaining server instance that is the witness in all its mirroring sessions. After an upgraded witness rejoins a mirroring session, automatic failover becomes possible again. You might have to update multiple servers at this point.”
I agree with Faisal on this
me 2