system upgrade for 3.2 ghz quad-core intel xeon mid 2012

Hi there,

I am using 3.2 Ghz Quad-core Intel Xeon mid 2012 with 10gb 1066 Mhz DDR3 ECC, my adobe software needs higher os (10.9 or higher) for program upgrades, Computer technician told me I can not upgrade unless there is a system upgrade option in applestore. Unfortunately I find no system upgrade in Applestore. How can I upgrade my system from 10.7.5 to 10.9 or higher? A step-by-step explanation is appreciated since I am not good at this os things.

And also will there be a problem with my fonts, opening indesign documents etc?

thank you.