Hello,
If it's been less than 10 days since you've upgraded to Windows 11, you can go to Settings > System > Recovery > Go back which will allow you to rollback to Windows 10 without losing any apps or files.
However, if it's been more than 10 days since you've upgraded to Windows 11, you can't use the method above; you must clean install Windows 10. This will unfortunately mean that you will lose your files and apps, so make sure to create a backup of your files beforehand.
One extra note: Windows Setup will prompt you and ask whether you'd like to create a Windows.old folder (which only appears if you don't format the drive). You can use this to recover any files that weren't backed up.
Hope this helps.