I want to upgrade macOS from Yosemite 10.10.5 on macBook pro early 2015 but want to keep functionality of of some of 32 bit apps

Back up your data, check your applications for compatibility, and download Mojave.

(181525)

Well as you know Catalina does not support 32 bit apps.

You can upgrade to Mojave or High Sierra.

Open your Safari browser and click on these links. Firefox does not work.

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says Open then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.