It's certainly within the rules to modify your itinerary after the visa is issued. The problem is that it is of course not acceptable to lie in your visa application, and sometimes people who change their itineraries are suspected of having lied in the application.
Adding a couple of days and an additional destination to a 13-day trip is not likely to raise suspicion, but in case it does, the more evidence you can muster to show that the change really did arise after you submitted the application, the better.
This of course assumes that your visa's period of validity is sufficient to cover the new 15-day trip, and that the "duration of stay" is also sufficient. For the duration of stay, be careful about counting correctly: both the date of entry and date of exit count, so you cannot calculate by simply subtracting dates. If you enter on the first day of a month and leave on the 15th day of the same month, that counts as 15 days, not 14.