To switch the Slack team you're logged into with Tettra, first, log out of Tettra.
That'll bring you back to the Tettra sign in page. Press the fuschia button.
If you're signed into Slack with multiple accounts, you'll see a list of teams to choose from. Pick the team you want to log into.
If instead you're seeing a screen like the one below, that means you're signed into only one Slack already and you need to change teams by clicking the "Change Teams" link. Then enter the correct team url on the next screen.