How can I upgrade OS X El Capitan 10.11.6 to El Capitan 10.12 on my Mac

Since the upgrade to High Sierra I am unable to access certain websites (ie: bank) and when placing an order I was kicked out at the "finalize and complete" point. I keep receiving the message: A problem repeatedly occurred with "the website" Reload Page. However there is not a problem with the websites as I am able to access them on my iPhone.

Yesterday I updated the High Sierra upgrade in hopes that it would help, which it didn't.

It is important that I'm able to access these websites, since I'm now primarily working from home and are necessary.

Thank you and I look forward to your response

Thank you! It worked perfectly!

Thank you....I'll try that.