How to Change Weight?
You can change the weight by tapping on it on the log workout screen. Just open the next workout, tap the weight or body-weight, and change things. Then to remember the value, enter at least one set. This marks the workout as ongoing and it will remember the new value you entered.
If you've trained before, Mehdi recommends starting with about 50% of what you could lift for 5 reps on that exercise.