Unable to update macOS

Currently running OS X El Capitan version 10.11.6. H&R Block software requires minimum version of macOS be 10.13.6 or higher. I haven't been updating to the newest macOS because each update slows my iMac down. However, I am now willing to upgrade to 10.13.6 as required by the H&R Block software. Problem is that I can't install 10.13. I've downloaded the .iso and ran the install and it appears successful, but the OS is never updated. Do I need to update to 10.12 and then 10.13. According to the macOS I should be able to upgrade directly to 10.13 from 10.11.6. Any help with finding and installing the correct ISO file would be greatly appreciated.