OS X-version 10.11 MacBook Pro (13", medio 2012)

Hi Appel experts

 I haven't used my MacBook in a few

years. Now I have started using it again. I have a problem downloading from the

appstore. My Macbook tells me I need to update my OS X version 10.11. How can I

do that? Normally my MacBook tells me automatically that there is an update,

unfortunately I have not seen any update called OS X version 10.11.

Can you help me update my MacBook? 

I have a MacBook Pro (13 ", mid 2012)