New iphone8 cant restore backup of iphone6s (12.4.1) using itunes

I purchased a new iphone8 last week from the apple store in sydney, and backed up my iphone6s which was on 12.4.1 using the latest itunes onto Windows10 PC on 19th Sept. When I transferred the SIM to the new iphone8 turned it on, then got to the step of connecting to the PC and chose option Restore backup, following the apple support document https://support.apple.com/en-au/HT210218

Then I got the message 'itunes could not restore the iphone... because the backup was not compatible with the iphone that is being restored, You must update the iphone to the latest version of iOS before it can be restored from this backup'.

I assume that means the new iphone8 I was sold had either 12.4 or an earlier release of firmware 11.x on it and so how do I put 12.4.1 on it now (the same as my iphone6s backup ?). I cant find a support document to tell me how to do this, or a download page for 12.4.1 iOS; should I return to the apple store and ask the person there who offerred assistance when I collected the phone to do that?

The iphone8 doesnt have a SIM (it's in my old phone) so I connected it via USB cable to the Windows 10 laptop , but if I update it via iTunes won't it put on iOS 13? Then I would not be able to restore my 12.4.1 full backup of the old phone. Any help much appreciated. I would like to be able to get my new phone set up to use with everything restored from the old phone.