Hi Bledar,
I have also had this problem in the past when I bought a laptop on the cheap and needed to upgrade from home to pro.
This one command has been my go to for solving it:
cscript c:\windows\system32\slmgr.vbs /ipk xxxxx-xxxxx-xxxxx-xxxxx
You need to run that command from an elevated command prompt.
And you need to replace the xxxxx-xxxxx-xxxxx-xxxxx part with your actual product key, which you should have gotten from the purchase.
After running this, it should give you a pop-up saying that they key was applied. and you can then reboot to see if it has worked.
If all fails, then a refund should be a fair option. I don't know if you purchased through the MS Store or a third party, but they all have refund processes that should be highlighted somewhere on their website.