How to Change Weight
You can easily change the weight from the log workout screen.
Just log a new workout and then tap the weight. You'll get a popup that allows you to change it. You can tap the weight itself there so the keyboard popups. You can enter whatever you want then with your keyboard.
If you've trained before, Mehdi recommends starting with about 50% of what you could lift for 5 reps on that exercise.
Do not make the mistake of starting too heavy. See this