If you got no budget and you're looking for a good performance go with the MSI GeForce GTX 970
If you got a strict budget I'd suggest something like Radeon R9 380.
Be aware of this important factor though. Since your PC is pre-built it probably has strict rules when it comes to upgrading, one of which is power usage. The graphics card that you're using now has 25 W power consumption, and for example GTX 970 has 145 W. What this means is if you're going to upgrade your graphics card, you will most likely need to replace your power supply to something more powerful as well.
Also be aware about the form factor and dimensions of the GPU/power supply when buying, otherwise you might end up not being able to fit the part inside the case.
I hope this helps, if you got any more questions feel free to ask :)