hi dev how to backup the wallet?

You will find the "wallet.dat" file in C:\Users\%username%\AppData\Roaming\ZettelKasten. All you need do is copy this file and save is somewhere safe (AFAIK).
Another method is dumpprivkey and save them somewhere safe

how to do this

In the zettelkasten-qt app, go to the tools->debug console menu and once the console opens, type "dumprivkey
" to get the private key of an address.
Thanks for the tip, I did this as extra backup.
There also is the console command "backupwallet " , after some testing I found this command does not like spaces for the backup destination, so 'C:\zettelbackup\filename' will work but 'C:\zettel backup\filename' will not work.
Correction:
Must have been half asleep, noticed it will only save to root of the selected directory and renames the file with any extra directory paths, thus..
'backupwallet x:\folder01\filename01' will result in backup saving to root 'x:\' with filename 'folder01filename01' and not save to '..\folder01' with filename 'filename01', why ?