Baritastic uses your earliest entered weight as your starting weight.
To add a new starting weight, you can click the green plus button, then click on weight. You can change the date by scrolling on the day and using the arrows to change the month. Enter a weight for a previous date before all other entries and click save.
To change an existing weight, click on the Chart button. Click on any previous weight and change them. The pencil icon will allow you to delete any previous entries as well.