New Transaction: While creating a transaction, scroll down to the Notes field, enter your note, and save the transaction.
Existing Transaction: Open the transaction, tap the ✏️ Edit icon next to Notes, update the note, and save.
If it's a recurring transaction, you'll be asked where to apply the changes:
This Only – Update only the selected occurrence.
This and All Future – Update the selected occurrence and all future ones.
All Future – Update only future occurrences.
For Accounts
You can add a note while creating a new account or update it later from the Account Details screen. Account notes are displayed only on the Account Details screen.