Older iMac from storage, cannot update OS and related

Hello Apple friends, I have an older iMac (late 2013) that hasn’t been powered up in maybe 3-4 years. Running OS Yosemite 10.10.5, 3.2Ghz Intel Core i5 w/8gb 1600 MHz DDR3 ram. Hard drive has 80% free space. I need to use this machine for a project since my phone just isn’t going to work. Problem: tried updating through App Store, gives error message of “Safari can’t open the page Support.Apple.Com because Safari can’t establish a secure connection to the server”. I have cleared browser data and cleared history. The time is correct, but it will not connect to whatever in order to verify correct date & time. What steps should I take to update this relic and get it back in service? TYIA