Going back to 8.1 or getting an activated Windows 10 installation will both require you to do a new (clean) installation.
You will NOT be able to install 8.1/10 Core(Home)/Pro as an upgrade over your current install, as downgrading an Enterprise SKU is not supported.
Don't forget to back up any important files, installers and serials prior to the installation.
If your connection is too slow, you might also want to download this on a different machine, so that your download is not being canceled due to the automatic restarts.