Unable to update operating system and SSL certifcates

I have a Macbook Pro (15-inch, Mid 2009) operating OS X El Capitan (10.11.6) it's in good condition, still fast after a replacement battery and hard drive was fitted a few years ago. I have no complaints with the device other than the fact it's getting to the point where some websites are increasingly unsupported by it due to my subjected issue. It also does not seem I am able to increase access using different browsers, such as Chrome, Safari, Firefox. It is affecting my ability to update my own website, stream shows, and access trusted sites that my mac deems not secure despite them being safe and accessible via other devices easily.