Tap on Care Plan on the menu at the bottom of your screen.
If you've never set your health goal, you'll see the text "Why is your health important to you?
Tap here to set your goal". Tap the link and type your thoughts, then tap "Done" once you're done!
If you've set a health goal before but want to edit it, just tap on Care Plan and then just tap on your goal statement - the editing window will open and you can make and save your changes!