Add data for previous days
You can easily go to your Month view and scroll upwards to change data for prior days. You will then see three months back from the first day with data as well, in a grey colour. Click on any day to add data for that day, and watch the colours change.
This is how you do it:
Go to your Month view, and tap on the day you wish to add/change data for. This will open the Add Data page, here you can add/change any data you wish and then you can either swipe right/left to change the date or press Save. As you swipe right or left the new data is automatically saved.
If you have previously been using a fertility or period tracker, you can easily add your past data (temperatures, period, etc) in the app. Just go to the bottom of your profile tab and click on 'Sync with Apple Health'.