Importing from Slite
To to your Team settings in Slite and click Expand next to Back up.
You'll then see a button that says Back up now — click that to receive a zip file by email containing your content in the form of Markdown files.
Once you've received the file, head back to Slab and go to Import & Export in your Team Settings. Next to Files, click Import. Drag-and-drop or click to upload the Markdown zip file that was emailed to you.