You can only go back to the version of Windows since your last upgrade.
After that you cannot simply switch back to the upgraded version. You will have to do completely new upgrade to Windows 10 with the risk that the period available to upgrade to Windows 10 at no cost has already expired.
Also Windows 7 is less secure than Windows 10 and End Of Life in 2020.
I recommend to stay at Windows 10 and delete your previous version of Windows to get a lot of disk space back.
Your question is off-topic for this forum by the way, try the Windows 10 Technet forum for other questions about Windows 10.
Danny