If you downloaded an ISO, you can burn to a DVD and reuse. If you do and upgrade a qualified Win 7, or 8.1, you won't need to buy a license, otherwise you will. If you have the ESD file from the download/upgrade, you can convert that to an ISO, then proceed. Sorry for all the "ifs", but they're necessary since you left out these details.
I just downloaded and installed W10 on 1 of my 2 computers. Thought I could copy the download file to a USB stick and take it to the second computer, but now realize I should have used the downloader. Is there a way I can avoid a second download to do this? I have to pay for the GBs.