That depends on several things:
1. If you bought the computer new with Windows 10, then you would need to find and purchase a Windows 7 or 8 license. They are hard to find and Microsoft is already updating those two to automatically receive Windows 10. If you go this route you would also need to make sure that you got all of the appropriate drivers for whichever version of Windows you move back to. This might cause you to lose some functionality.
2. If you upgraded to Windows 10 from 7 or 8, if it is within 30 days you can roll back to your previous version of Windows through the settings menu, then Update and Security, and then Recovery.
3. If you upgraded to Windows 10 from 7 or 8 but it has been longer than 30 days then you would need to see solution 1.
I hope this helps.