1 - First go to the board you want to export and click on settings and then "Export board".
2 - After that select format "CSV" and click to export. You will download a CSV file.
3 - After that go to the board you want to import the content into, click on settings and "Import CSV".
4 - After you click, a popup will open and you need to click on "upload a CSV" to select the CSV file you downloaded before.
5 - The final step is to match all the columns from the CSV into the new columns of the new board and click "Import".