How can i update iMac os x el capitan 10.11.6 to the current os?

That would be a function of your iMac's market introduction date, and if it meets the Catalina (being the current operating system) hardware requirements. From menu : About This Mac, the market introduction date will be in parenthesis on the Mac product description line. Compare that date (e.g. Late 2013) to the requirements in the following:

How to upgrade to macOS High Sierra

How to upgrade to macOS Mojave

How to upgrade to macOS Catalina

What model Mac do you have, exactly?