Unfortunately there's NO option to go back from 8.1 to 8 other than a full factory reset, which will lose any programs and data you've put on the system since buying it, so first you need to back up anything you need.
The process for doing it will depend on the make & model, but most manufacturers include a Recovery utility (which should be accessible from the Start screen) to do the job, alternatively others will require a F key (function) keypress during boot or similar to allow you to initiate the factory reset process.
Edit: For anyone else with Windows 8, the update to 8.1 usually appears as a banner across the screen - you can dismiss it, and there are ways to stop it permanently if you want to take that route.
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