What upgrades are possible in this system?

Sadly Apple's laptops don't have removable chips (thin laptops tend not to). Here's a view of the logic board:

The larger green chip is the CPU

There are two models in the Mid 2012 (2.5 & 2.9 GHz) the only choice here is a logic board swap out if you have the lower end model within the series you could get the upper end model but it’s not enough benefit unless your logic board had died. Likewise, there is no newer logic board possible either as Apple completely redesigned the MacBook Pro in 2012 with the retina series. So upgrading is not possible to anything newer.

But! You do have some areas which you can gain performance!

To start with, I would bump the RAM up to 16 GB.

The next area would be the storage. Here you could swap out the HD with something bigger, I would go with a SSHD from Seagate as the best option here. The other option here is to take the big leap to a SSD. Just make sure you get one that has enough space. After you load it up you want to have 1/3 to 1/4 of the drive free more for the smaller SSD's.