Is it possible to upgrade the CPU on a mac mini?
"The soldering is very difficult" is rather an understatement.

The CPU architecture is BGA [Ball Grid Array] which requires a special machine to replace.

If all you have is a soldering iron, you can forget it.
eBay has BGA rework stations from about $£€ 400 to 4,000.

Once you've got the gear, then you could watch this YouTube video to see how easy it is… https://www.youtube.com/watch?v=GK62KML0D04