As long as 30 days has not passed since you installed windows 10, then follow the below steps to uninstall it:
Note: It will only work if it’s been less than a month since you upgraded to Windows 10.
1. Open Start menu and click on Settings
2. Click the “Update & security” icon and from there select “Recovery.”
3. Click “Go back to (Whatever OS you had)”
4. Click the get started button, and then you will be guided to rest of the process.
5. That’s it 🙂