Iphone backup

I have 2 devices assigned in my iCloud account which is already have many data.

I need to take my device to apple workshop for repair & they advised me to back it up.

Now i want to backup one of these devices & restore back the same data that was in it, not the whole date on my iCloud account.

How can i do it without overwrite the old data or mix them all up in the process of restoring the device ?