turbotax 2020 requires 10.14.

I recommend upgrading to Mojave and stopping there for now. Catalina makes a number of older apps stop working. Example: versions of Office older than Office 2016 will not work, and 2016 will not work unless updated, preferably while still running High Sierra 10.13.

This give yo u time to determine what older apps may go away, anore more time to update or replace them.

A couple of upgrading tips:

1) Before upgrading using Niel's links, look in your Applications folder for any older macOS installers that may have downloaded automatically. Remove them to prevent the process from erroring.

2) Apple system downloads are best done using Safari. There are many reports here of FireFox not working for that task.

More info here:

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

(Don't know how to do that.)

No worry--it's easy. As tbirdvet says, an external USB3 drive and Time Machine solve the problem, and Time Machine is already on your Mac. Beyond that, you attach the drive, then open System Preferences > Time Machine.

Activate TM by turning on "Back up automatically" and use the "Select Disk" button to choose your external drive. The first backup will take a while so be sure the MBP is on the power adaptor.

Apple article here: Back up your Mac with Time Machine - Apple Support

Back up your data, check your applications for compatibility, and download Mojave or Catalina through the links provided.

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You should get an external drive and setup Time machine that will backup up everything before proceeding.