Trouble updating i Mac OS from High Sierra to Catalina - Help

I hope someone here can help me. I have a 27' iMac, Late 2013 model, and am currently running it on High Sierra OS. I am unable to use a specific photo editing software because I need to upgrade to Catalina OS. So I went to Apple support and downloaded an updated Catalina. I open the .pkg file but I get an alert which says "This update requires macOS version 10.15". So I go back to the site, try to look for 10.15 sharp, but its nowhere to be found. I request an apple support ticket but am given an error message. So I thought, maybe if I update to Mojave first, then update to Catalina. Look for Mojave 10.14, nowhere to be found, download a more recent version anyway & same thing happens. I google download Catalina 10.15, and found a couple of sketchy looking sites, I download a zip from one, and get an alert from my antivirus, so I cancel. I already checked the system requirements and my iMac does indeed support Catalina. Can anyone guide me on how to be able to install Catalina?