Would it be better for me to take the computer somewhere to have this done?

ChiefOley wrote:

Can I partition my hard drive to keep macOS Mojave and The Printshop 4 on that part of the drive?  Then install the newer macOS system on the other part with all the other programs.  

Yes. You can create an additional partition on the drive and install the newer macOS there without affecting the other partition with Mojave and the PrintShop app.

Then build a startup page that allows me to start the computer in either macOS Mojave or macOS Other?

This is done automatically by the system when holding down the command + r key during startup of the Machine.

Would it be better for me to take the computer somewhere to have this done?

If you don't feel comfortable using Disk Utility to partition the drive and installing macOS, it may be an option to pursue. It should not be to difficult to do, but it does require some level of understanding of what you are doing.

You can boot up to Recovery, by holding the option + r keys on your keyboard when turning on the Mac. Click here ➡️ Mac startup key combinations - Apple Support

With that said, I would strongly recommend you backup the Mac before proceeding in any fashion, as there is a good chance the existing partition can get erased or damaged in the process.

1. Yes. Note that if you use Boot Camp, this may disable it.

2. Restart with the Option key held down or use the Startup Disk pane of System Preferences.

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