Click on the 'Manage' button - next to the division on ADMIN HOME.
On the Teams tab, find the team and click on the 'Remove' link.
There are options on the next page on what to do with any results entered for the team and upcoming matches.
There is also a replace team option on this page.
Replacing a team is different to renaming a team. To edit the name of a team (this will be reflected across all seasons):
Click on the 'Manage' button - next to the division on Admin Home.
On the Teams tab, find the team and click on the 'Edit' link.