There are in fact 2 points here:
- can an aircraft computer be hacked?
- is it really a question?
For the first question, AFAIK, the airplane only exchange trivial data with the ground and there is no distant direct interface. So it can only be hacked if you have a local access
For the second one, if you have a local access you can do much more damage than hacking the system: the system is programmed to react according to various sensors giving its air velocity, air pressure, etc... If you replace the velocity sensors to return wrong values past a time it could lead to terrible behaviour. And it you were able to do that, you could just install a little bomb. Both simpler and more efficient...
That being said, providing wrong parameters from the ground can also be very dangerous. The pilot is expected to know correct the air pressure at the landing point. If a wrong (but not stupid) value is entered, the system could try to land 100 meters below the ground. If visibility is poor a crash is to be expected...
Ok providing only one wrong parameter should not be enough because aircraft systems use a lot of redundancy, but providing multiple and consistent parameters could be really dangerous. Fortunately, it soon becomes very hard to do.