How do I transfer Windows 10 license to a new PC?

dalchina said:

...Showkeyplus may help to recover the Win 8 license from the old one... - assuming that's still functonal...

It doesn't even need to be functional, just the HDD will do. Mount it in an external usb case so you can access it then click 'Retrieve key from backup' at the top of ShowKeyPlus. Navigate to Windows\System32\config on the external drive where its SOFTWARE hive will be found. ShowKeyPlus can read the key from that.

DigitalPixel said:

This is a retail version. I don't know the product key. I've been searching for some kind of receipt but it's been years.

You can hardly prove it is a retail version when you have no product key and receipt.
I had to enter the product key after a hardware change last year.