how do i retrieve a deleted note

You can use iTunes on your computer to restore from your latest backup. Simply plug your iPhone into iTunes, do not allow it to sync when you do so, and then click "Restore from backup" from the summary panel on the right.

Here's another (hopefully more helpful) option...

If you're using Gmail to sync your notes you can retrieve them from the trash/bin in your gmail account. You may need to log into gmail via a browser on a desktop to get the full feature set for the gmail interface (google offers only a stripped down interface for mobile devices) & you may have to trawl multiple trash/bin/deleted-items locations (depending upon what other email clients have done to your gmail account). You will be able to retrieve notes synced to gmail for up to 30 days after deletion.

Something like this may also work for iCloud, but I've not had the opportunity to try it (I just retrieved my partner's deleted notes via gmail, tho, so I know it works for gmail-synced notes) -- YMMV.

While restoring from backups is great, but it ain't gonna work for the notes that are created a couple of hours ago & then deleted from gmail because the mailbox was getting unwieldy.