You can turn off automation from the Workflow page, or from the main Automations page.
First, navigate to the Workflow page by clicking on the name of a campaign. From the Workflow page, switch the toggle in the top right corner to Off.
From the main Automations page, click the Menu icon at the end of the automation's row. Then click Turn off the automation.
You will now be prompted to either "Stop automation runs in progress" or "drain automation runs". By selecting to stop runs in progress, all scheduled steps will be stopped immediately.
By choosing to drain the automation, you are allowing the remaining steps of the automation to run for any account still in progress, however, no new runs will be activated.