According to the current models, the possible reasons for a supernova are thermal runaway or core collapse.
Thermal runaway is probably your best bet, and happens when the core of the star gets hot enough to start fusing carbon. This usually happens when a white dwarf steals enough mass from a companion.
Core collapse happens when nuclear fusion becomes unable to sustain the core against its own gravity. This usually happens in very large stars when the core gets enough iron to stop fusion. Stars cannot fuse anything heavier than iron, and so once you get to that phase fusion stops. This allows the core to collapse and either become a neutron star or a black hole, depending on the starting mass and composition. The process of collapsing also causes runaway fusion resulting in a supernova.
So how can the aliens do either of these things artificially?
The easy way is to get a large amount of energy into the center of the star which would trigger runaway fusion.
Peter F Hamilton used a Nova Bomb which used some kind of quantum fission effects to turn a jovian sized chunk of the star into energy.
Another way would be to deposit a quantity of anti-matter into the star's heart which would have the same effect.
Delivery would be difficult, but not impossible.
Both of these would require a level of technology way above what we have, but are not out of the realm of possibility.
As to Core collapse, this is a little harder, having to introduce a lot of iron into the star. The amount needed is called the Chandrasekhar mass, and is a lot of iron. If you could find an iron star and wormhole it into the core of an existing star that could do it, but the universe is far to young to have any iron stars yet.
You could transmute some of the stars mass into iron, but the energy needed would probably cause runaway fusion long before you could get the core to collapse.
The Charles Stross book Iron Sunrise used a temporal bomb/effect on a star to cause the core to age thousands of times faster than the rest of the star, causing it to fuse into iron very quickly, which collapsed the core and set off a nova.