2 ways, the device manufacturer provides the OTA (Over the Air) upgrade and you'll see it when you check for updates in Settings > About, or you need to find someone that's built a custom ROM for the device then root it so you can install a custom recovery and the custom ROM.
I have rooted my I ball andi 5li but I didn't find any update option on about & I have search ROM for me but I didn't found plz help I want to upgrade my phone
You could always create your own ROM. Just because you rooted the device won't make an update magically appear. If the manufacturer didn't develop an update for the device, you'll need to rely on the ROM community to develop one. If that never happens, you won't see an update beyond your current software version.
How do u upgrade, i've looked and looked for the apps and all I find is a "test", frustrating when it says its the upgade and it's not. Yes, I check for updates in my settings and all I ever get is a message saying "temp. Down.