If you're running 7 now, do you have a copy of the original license key? If it's a brand-name laptop or desktop that came with 7 there should be a sticker somewhere on it with that key. As long as you have the key you can do a fresh install of the software (available from the link below, which you can copy onto a prepared 8GB flash drive). Until the free upgrade offer expires, valid 7 or 8.1 keys can be used to activate a new copy of 10. Make sure anything you don't want to lose is backed up first.
download link:
https://www.microsoft.com/en-us/software-download/home
To prepare flash drive, the basic idea is format as NTFS and set "active" flag
You can do it at a command prompt (as administrator), typing "diakpart" to open a partition utility. "list disk" then brings up a listing of the drives with drive numbers. Enter "select drive x" where x is the number of the flash drive. The command "clean" erases everything on the drive, then "create partition primary" and "format fs=ntfs quick" format it (if the format command gives an error, you may need to "select partition 1" first). Finally just type "active" then you can exit the utility.
From that point, just copy the contents of the .iso downloaded above (not the .iso itself but the unpacked contents!). Windows 8 and above can open .iso files natively, for windows 7 this utility lets you open the file in a virtual CD drive (from which you copy on to the flash drive).
https://www.elby.ch/en/products/vcd.html