*Only Team owners have permissions to do this*
1 - Go to the team name in the Teams list and click More options > Manage Team.
2 - Navigate to the Channels tab to bring up all the channels on the team. Click on the 3 dots to right of the desired channel & select 'Delete this channel'.
3 - You will be asked to confirm the deletion of the channel in a new pop up window. Once you're happy to proceed, click 'Delete'.
*Please note that you deleting a channel will result in the deletion of all files & folders associated with this channel, and cannot be undone.*