You have to be an Account Owner or a Workspace Manager of at least two Workspaces in your account to be able to copy or move boards from one Workspace to another.
1. How to Copy/Move a Board?
Go to the Dashboard, where all your Workspaces and Boards are located and select:
- the settings of the Board that you want to copy -> select Board settings to display the "Workspace and Boards Settings" panel.
or - the Workspace settings where the board belongs to -> select Manage boards to display the "Workspace and Boards Settings" panel
On the left side of the panel, you will find the list of all existing boards that belong to the Workspace. Make sure that you have selected the Board that you want to Copy or Move (1).
Open the Copy/Move Board section (2).
Copy a Board
In the dedicated "Copy/Move Board" panel, select "Copy" (1) and choose the Workspace (2) where you want to paste the Board itself from the dropdown list.
(3) You have the option to copy some of the board settings. Checkmark the ones that you want to preserve. Click the "Copy" button. 
Move a Board
In the dedicated "Copy/Move Board" panel, select "Move" (1) and choose the Workspace (2) where you want to move the Board to. Click the "Move" button.
If you select to move the Board, you will not be able to get the settings and you will move the Board as it is.
Please, check the dedicated article to learn how to copy a Workspace with all belonging boards.