I have an iMac mid 2007. what can i do to upgrade the system, It's now OS X Yosemite 10.10.5

Back up your data, check your applications for compatibility, and download El Capitan.

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You can upgrade OS X to El Capitan - OS X 10.11.6 (15G31). RAM can be upgraded to 6GBs (2GB and 4GB modules.) Put in a larger HDD or SATA SSD, if you need the space or more speed. But the best thing you could do to u upgrade your system is to buy a newer model. Your computer is now 13 years old.