Assuming you want to downgrade from Windows 10, you have 30 days to revert back to the previous version of Windows. If you miss it, then you'll have to perform a clean installation from your installation media.
Most off-the-shelf PCs have a recovery partition that puts the OS back to how it was when you bought the PC. You normally start this by pressing a function key during POST. Bear in mind that all of the data on the drive will be wiped in the process.