In this example, let's imagine you have two Milanote accounts (you@personal.com and you@company.com) and you want to move a board from one to the other.
Step 1. Share the board with your second account
Log in to the account which contains the board you want to move (e.g. you@personal.com)
Navigate to the board board you want to transfer.
Open the Add Editor popup and invite your second account (e.g. you@company.com) to edit the board.
Step 2. Transfer board ownership
Once your second account (e.g. you@company.com) has been invited to edit the board, open the "Editors" popup again.
Select the three dots icon next to your the you@company.com account, then select Make [User] the Owner.
This will transfer board ownership over to your second account.
Finished!
Now you can log back in to your second account and use your boards as normal.