So i want to replace my motherboard

do you have a M$ account?  if so, you can associate your free (if this a free upgrade version) key with that account.

If you haven’t yet signed in with a Microsoft account, head to Settings > System & Security > Activation and you’ll be prompted to add a Microsoft account to make reactivation easier.

Once you’ve added a Microsoft account, you’ll see the “Windows 10 is activated with a digital license linked to your Microsoft account” message here.

When reinstalling Windows 10 after a hardware change–especially a motherboard change–be sure to skip the “enter your product key” prompts while installing it.  If you’ve just changed a few peripherals, Windows 10 may just automatically activate itself after you do a clean install. But, if you’ve changed the motherboard or just a lot of other components, Windows 10 may see your computer as a new PC and may not automatically activate itself.

Head to Settings > Update & Security > Activation and you’ll see a “Troubleshoot” option if activation failed. Click that option and sign in with the Microsoft account you associated your license with. You’ll be able to tell Windows that you “changed hardware on this device recently” and select your PC from a list of devices associated with your Microsoft account.

To clean-install Windows 10 on a machine activated in this way (via free upgrade), you have to continually skip all the product key prompts while installing it.