This and other workouts you'll find here have been compiled using sports science knowledge. The number of repetitions in an exercise is also calculated and specified with your training target in mind. Select the weight so that you can manage this amount of repetitions – no more and no less. The last repetition should really be the last. If you can manage more repetitions, then you have selected a weight that's too light. If you can only manage a few repetitions, then the weight is too large. You should only change the amount of repetitions during the workout in these two cases. Try to achieve the specified amount of repetitions the next time you do this exercise.
For exercises using your own body weight, it's more difficult to measure the stress. The repetitions that you manage until you hit your target count here. If you can do more repetitions than the target, try to use additional weights or do the repetitions slower and with more tension, or incorporate intensity techniques.
For certain exercises it is worth holding the position for a certain (stress) period. If you can't manage the specified time, take a short break and assume the position again until you reach the overall indicated time.