So basically, you'll need to reach stage 3 in Economy & Trade (on the right) from the Technology Screen. You 'll the unlock the capabilities to create System Upgrades from the Economy Screen (Trade & Resources tab).

You'll have to choose a Luxury resource for the upgrade (choose wisely, you will not be able to change it after it's done !).

Finally, you can build the upgrade your systems, using 25 units of the luxury resources you've choosed.

You'll unlock more upgrade with the next stages of the Economy & Trade researches.

Hope it helps :)