To switch workout guides, just go to the "Plan" tab and swipe left or right to browse through the available options.
You can start a guide by selecting your preferences (e.g. workouts per week, experience level) and tapping 'Launch Plan' at the bottom of the guide page.
If you have already started the same guide in the past, you can restart it by tapping 'regenerate plan' on the top right. Please note that resetting a plan will lose any tracked sessions you had on it.