data is backed up on an extention drive connected to my laptop.
There is no need to import - just open using File - Open
There is a challenge
Copying between Windows and Linux often faces the challenge of unsupported filesystems.
Linux supports most file systems out there, but on Windows’ preferred file system, NTFS, only reading is reliable (not write). The one family of filesystems well supported by both Linux and MS Windows is FAT, so if you are lucky, the drive you backed up on uses a FAT filesystem (FAT32, exFAT).
There are solutions
At best, you have some kind of FAT-storage. You just plug it in, open the backup folder and copy the content to your documents folder. You probably already tried that …
If you still have the Ubuntu system running, the easiest way is to copy via some kind of cloud storage. Google disk and Dropbox used to work well for me on both linux and Windows, and I guess they still do. A network shared folder is also a common solution. You probably can’t do that …
From the wording of your question I guess you installed Win10 on the same computer where Ubuntu used to be, so the old content is wiped from there.
Going back to Ubuntu, or some other Linux, is your best bet. Do you have another computer with network access and Linux installed? A friend’s computer? You can create a SaMBa network share from the USBdisk and connect your Windows computer to that, or copy via cloud storage.
If you have sufficient disk space available you could also install a desktop virtualization environment (Virtualbox?) and install a working copy of Ubuntu there. Enable USB device support and “shared local folder” for the virtual computer and connect the USB disk in the virtual environment, so you can copy from backup to “shared local”.