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. Follow these steps...
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)
Open the board you want to transfer
Click the Share button in the top right
Invite your second account (e.g. you@company.com) to edit the board
Transfer board ownership
Once your second account (e.g. you@company.com) has been invited to edit the board:
Click on the avatars that appear to the left of the "Share" button
Find the account you'd like to transfer content to
Click on the arrow to the right of it
Click "Make you the owner". This will transfer board ownership over to your second account
Finished!
Now you can log back into your second account and use your boards as normal.