My Mac says it is up to date with V. 10.13.6 and does not give me any way to update it. I am trying to download iMovie and need a better updated version to download it. But no matter where I go it does not give me the option to update.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work

To check for 32 bit applications before upgrading to Catalina or above - suggest using a Trusted Developer GO64

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

to see if you can run a more recent macOS after 10.13.6 read the following apple support page:

How to get old versions of macOS - Apple Support

you will find if your mac meets the system requirements for a more recent macOS than high sierra.

It is up to date. You're confusing "update" with "upgrade." 10.13.6 is the last update for for High Sierra. Moving to a different macOS version is an, "upgrade," not an "update."

Get upgrades from the App Store app.

iMovie requires macOS 10.15.6 or later.