As others said just to replace it for efficiency may not pay back. But
you give to little information for help, location, yearly cost to
operate, system condition, system efficiency, house insulation, present
and anticipated Kwh cost and present sizing are important. To go from a
12- 14 seer savings will be not great , maybe 15%. Going to a 19.5 seer
might save 35-40%. The Trane website has a page "Efficiency and Savings"
for Ac units that I pulled those numbers from. Research and run your
numbers, that is the only way for you to learn and know.