Connect a USB flash drive with enough storage space. Open File Explorer, select This PC, and open your usual folders such as Desktop, Documents, Downloads, Pictures, Videos, and Music. Copy the files you want to keep and paste them into a folder on the thumb drive. If you use other folders where you store work or personal files, copy those as well.
If you use OneDrive, first check whether your files are actually stored locally or only in OneDrive. In File Explorer, look under OneDrive and make sure important files have been downloaded to the Surface before copying them. A cloud icon can indicate that a file is online-only. Right-click important OneDrive folders or files and choose Always keep on this device, let them finish downloading, and then copy them to the thumb drive.
Also back up things that may not be obvious from Documents and Pictures, such as files on the Desktop, browser bookmarks if they are not synchronized, downloaded installers or personal files, and any application-specific data you need. If you use Outlook with locally stored PST files, make sure those are backed up too. You can search the Surface for *.pst using File Explorer if applicable.
I would not rely on the thumb drive as the only copy. After copying everything, open several files directly from the thumb drive to verify that they work. If you have enough space, keeping a second copy on an external drive or cloud storage is even safer.
For the cloud based option, refer to https://support.microsoft.com/en-us/windows/experience/backup-recovery/back-up-and-restore-with-windows-backup
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hth
Marcin