If you have a valid Windows 7 licence you can use that to activate Windows 10 when installing. Windows 7 Home Basic / Premium = lets you install Windows 10 Home Windows 7 Business / Ultimate = lets you install Windows 10 Pro If you don't have a licence then you will need to buy one to activate. These are usually pretty cheap on Amazon, although be warned their probably from MSDN so are not supposed to be sold, so could be invalidated at anytime. However for a few £ might be worth the risk: https://www.amazon.co.uk/Windows-10-pro-key/dp/B07NWT3R41/
The free upgrade that Nutter mentions is still available and a great deal. Not easy to find retail versions of Windows 7 available these days, but I recently bought a refurbished machine for under £50 which came with official Windows 7 refurbished licenses and I could upgrade it to 10 easily. Look for any of these 'Microsoft MAR' sellers and you can pickup very cheap machines with legit software.
Most WIndows 10 updates only download if you tell them to do so. Depending on what build of Windows 10 you are running, this option may be on or off by default. I would turn off automatic installs, but still allow notifications.