You need to log in to your Workflow account first and go to the workspace settings by clicking on the icon mentioned in the image. The icon showing the first letter of your account name.
The workspace settings option is present. Click on that.
A page of Workspace settings will open up. In that, you need to go in Workflows.
Again, a page will open up showing all the workflows. You can activate or deactivate your workflow from this page as the options are clearly visible.
The workflow which you want to activate or deactivate, just click on the actions of either activate or deactivate. As soon as you click on the activate or deactivate button, a pop up will appear which ask for your surety. For example: I want to deactivate Leads workflow, so I will click on deactivate option mention in the action.