I have an iPhone 6 plus running iOS 12.5.1. When I go to software update it says it is up to date. How can I get it to update to 13?

marshinspace21 wrote:

...How about a mac pro that runs os el cap. Can I get catalina to run on it do you know?

That depends on which Mac model and year.

Mac Pro “trashcan” 2013 and newer are compatible.

The older “cheesegrater” Mac Pro models are not.

Supported Macs: macOS Catalina - Technical Specifications

Use 🍎 > About This Mac for the year.

And to extend the above (correct) answer, your options are a newer or new iPhone.

There are no hardware upgrades.

iPhone 6 and iPhone 6+ has half the memory of an iPhone 6s and later, among other limits.

Your iphone 6+ cannot run anything higher than what it has now. If you need something to run iOS 13 or higher you will need to buy newer equipment.