i would like to update my OS to Mojave but it tells me it can't update to my computer. It is a late 2013 iMac.

Using Safari click on the "macOS Mojave 10.14" below.

Safari uses these links to find the old installers in the App Store. After downloading from the App Store, the installer opens automatically.

• macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Mojave 10.14 can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion
• macOS High Sierra 10.13 can upgrade Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion

Please refer to How to get old versions of macOS to learn how to get Mojave. Also, please note that a 2013 iMac can upgrade to Catalina 10.15.7 but that is as far as it will go.