You can easily go to your Month view and scroll upwards to change data for previous days.
The very first date with any entered data will be shown on your app, along with 3 months before it in a light grey color. Click on any grey day to add data for that day, and watch the colors change.
You can do this by:
Going to your Month view, tap on the day you wish to add/change data. This will open the Add Data page, here you can add/change any information you want to 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 saved automatically.
If you have previously been using a fertility or period tracker, you can easily add your past data (temperatures, period, etc.) in the app. To do this, you can go to your Profile tab > Manage your data > 'Sync with Apple Health.'