How do I update from version 10.13.6 to a higher version

Back up your data, check your applications for compatibility, and if your iMac is a 2012 or newer model, download Mojave, Catalina, or Big Sur. If not, you can't.

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If you mean it is time for a new iMac, then if you can wait a few months, and see if Apple releases an Apple Silicon (M1 or M1X CPU based) iMac, you will be better set for the future.