The answer is that it's very simple to uninstall Windows 10 and go back to your previous version of Windows (meaning 7 or 8.1). Microsoft thoughtfully made provision for this and the process usually works well. The caveat is that the uninstall option is only available for 30 days following the upgrade to Windows 10; after that period Microsoft makes the reasonable assumption that you are happy with it.
Before you proceed, make sure you have backed up your important data, just in case things go wrong. Then, click Start followed by Settings. Go to Update & Security and choose Recovery. If you are eligible to downgrade, there will be an option to do so called Go back to Windows 8.1 or Go back to Windows 7. Note that the process takes some time and once the computer is back on the previous version you may need to tweak your system and/or reinstall some programs.
Now, suppose you change your mind yet again and decide to give Windows 10 another chance at a later date? Again, no problem - you can download and install the free Windows upgrade as many times as you like up to 26th July 2016 (thereafter it will become a paid option).
Note that process is only applicable if you have upgraded a machine to Windows 10 from an earlier version. If you buy a new computer that already comes with Windows 10 there is no option to downgrade it to Windows 7 or 8.1. Also, if you were part of the Windows 10 Insider Preview program there is no option to do so either.