Upgrade from High Sierra to Catalina

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.

And in your case choose Catalina

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

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, Defrag, VPNs 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 expand on P Phillips wise inclusion of the 32/64-bit issue: That means that if you have older versions of MS Office, expensive Adobe software like Creative Suite that are an older versions, or old software that cannot be replaced (like many older games), you may wish to stop at Mojave 10.14.

Office 2016 will only work with Catalina if you fully upgrade Office before going higher than Mojave. Office 2011 and older cannot run at all on Catalina or Big Sur so will need to be repurchased.

If you are not certain which apps now on your computer are 32 and 64-bit, this free tool is quite helpful:

https://www.stclairsoft.com/Go64/index.html

Yes.

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