How to add resolution to Mini

If this is a duplicate - please send me the link for it as I couldn't find any..

I am using an older Mini (Mid 2010) 2.4 GHz Core 2 Duo

2 GB RAM with GeForce 320M 256 MB

I just updated to OS X Yosemite v10.10

Now, looking up the 320M, I find that it's max res is 2560x1600.

My work monitor is 2560x1440, but my current resolution is somewhere lower than than - perhaps 1800 or something...but there's a noticeable difference with my Windows PC monitor (running right besides it) at 2560x1440.

I noticed, when installing OSX, that you can choose what drivers to install, and what resolution to choose then.

So how, on an existing install, do I add drivers to support the higher resolution? I hope I don't need to reinstall from scratch??!?

Thanks,

Rob