To register that you watered your plant a few days earlier than scheduled:

1. Mark your plant as watered 
2. Tap the finished task 
3.  Tap "Add details" > "Completed date" > set the date back to the correct watering day 

Note that this only works if you have completed the task today, you can’t change dates on previous tasks. It is also not possible to change the date to before the plant was added in your account.