It is not simple.
It seems to me that it is not possible to find Safari 6.0.5 for legal download on the internet.
At one time, it was possible to find the installer for an application directly from Apple's website in the support / download section. Now, applications and updates are downloaded directly from the Mac App Store, we no longer have access to the installation files.
[Note : Over time, Apple locks us into its ecosystem and makes us, more and more, hostage to its cloud computing. Apple dispossess us of our applications, take our data (Calendar, Address Book ...) and makes us dependent on its Cloud].
In my case, I always have two backups on two different external drives.
These backups are made with Carbon Copy Cloner, which are clones of the internal disk of the iMac, which is a volume Fusion Drive here.
From a backup, maybe is it possible to simply put in the trash Safari 6.1 and replace it by Safari 6.0.5 from backup. I have not tried it.
Personally, I backed up my iMac just before the update Safari, so I restarted on an external backup drive, I deleted the internal disk of the iMac with Disk Utility, then I transferred my external backup to the internal drive with Carbon Copy Cloner.
For 300 GB of data, it takes about 2 hours by USB3. I started the transfer before the lunch break. Upon my return, my iMac was ready.