Open the Sidebar (swipe from the left or tap on the hamburger menu to the top left)
Click on the Settings icon (gear icon to the top right of the sidebar)
Open the connection that you want to delete. Click on the "3 dot menu" in the top left and then select "Delete connection".
When you delete you will be asked if you want to KEEP or REMOVE the transactions imported by that connection.
The transactions previously imported by that connection will remain as is and the connection will be deleted i.e. it will not import future transactions.
It will remove ALL transactions added by the connection. This action is irreversible! If "Remove existing transactions" was enabled when adding that connection, this will also restore the manual transactions that it had removed. This way adding Exchange Account Connections are reversible (as a whole) if you are not happy with the import overwrite.