Sticky notes are stored under C:\Users\Your user name\AppData\Roaming\Microsoft\Sticky Note
Convert StickyNotes.snt to plum.sqlite
To convert old Sticky Notes StickyNotes.snt data file to the new Sticky Notes plum.sqlite format do the following:
On your Windows 10 machine, close Sticky Notes.
Now open Settings > Apps > Sticky Notes > Advanced Options. Press the Reset button. The app will be reset to default, and all app data will be deleted as well.
Do not open Sticky Notes app now. Instead, open File Explorer and navigate to the following folder:
%LocalAppData%\Packages\Microsoft.MicrosoftStickyNotes_8wekyb3d8bbwe\LocalState\Legacy
Here you will see StickyNotes.snt file. Rename it to ThresholdNotes.snt.
NOTE: If the Legacy folder does not exist, you will need to create this folder here in the LocalState folder and then place the StickyNotes.snt file from your older Windows system here in the Legacy folder. Hopefully, you will have copied or backed up this StickyNotes.snt file from your earlier system.
Now start the Sticky Notes app. The data in the .snt file will be automatically transferred to a new .sqlite data file.