Generally speaking (and for safety reasons), you would do more sets/reps. It is recommended to complete the entire recommendation first and add a set with more reps if you are able to complete the exercise with proper form. Please keep in mind that the goal is to not "max out" or completely fatigue after the first set as you'll most likely have more sets to go (additionally sets can catch up to you quickly).
Alternatively, if your second to last set is still too easy, you can increase weight slightly and finish the exercise at that weight.
If you do add more sets/reps/weight, the app will account for this in your recommendations for the next time you perform the exercise.
Please keep in mind that this is a general response and if you have a specific goal of lifting heavier weight for competitive purposes this advice might not pertain to you. For additional information or questions, feel free to reach out to Ask a Trainer program by sending an email to firstname.lastname@example.org.