You must be the Board Owner to transfer ownership of a board.
Board owners have a different set of permissions on a board and control all of the settings on a board.
Learn more about Board Permissions
The creator of the board defaults to the Board Owner. Boards can only have one Board Owner, however ownership can be transferred. Only Board Owners can delete/archive a board or make another board member an owner.
Changing an existing member to a Board Owner
To make a board member an owner, click their avatar in the board member's menu in the top right corner of the board.
Select the board member and click "Make Board Owner"
By transferring ownership to this user, the following applies:
Your board permission level will be downgraded to BOARD ADMIN
You cannot reverse this unless the new Board Owner transfers ownership back to you
Your screen will reload upon making this change
Confirm the transfer by clicking TRANSFER