I have a mac pro with os x 10.9 which should i upgrade to to update my mac

That would depend on your Mac Pro, is it one of the big silver towers, or is it the dustbin.

The silver towers (cheese graters), 2008 and 2009 can only be upgraded to El Capitan.

The 2010-2012 can upgrade to mac OS High Sierra or if they have a Metal capable graphics

card they can install macOS Mojave.

The 2013 dustbin can upgrade to all OS's up to Catalina.

All the installers below are available for download.

Open the Safari app, Firefox does not work and click on each of these links

and go to Section 1 in each case.

How to upgrade to macOS Catalina – Apple Support

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to macOS Sierra - Apple Support

How to upgrade to OS X El Capitan – Apple Support

How to upgrade to OS X Yosemite – Apple Support

Thats not a MacPro then, that will likely be a MacBook Pro.

What does it say under >About This Mac.

2007 or 2009 models will be restricted to upgrading to Mac OS X El Capitan.

If you want install El Capitan click on the link below.

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan,

a mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

The Mac App Store undergoes a substantial change at 10.11 El Capitan.

Even if your Mac is qualified to install later version, you should Install El Capitan as a waypoint, because it will make installing a later version much easier.

Those are likely limited to El Capitan as the newest MacOS supported.

If you have less than 6 GB RAM, your Mac may not reposed in an appropriately-responsive way.

 Menu > About this Mac ...

will reveal the model information (but take care NOT to post your serial number)

The Mid 2012 model can be upgraded (by way of El captain as a waypoint) up to the latest MacOS, which at this writing is Catalina.

As long as it is NOT a Retina display, you can upgrade the RAM to as much as 16GB using up to an 8GB DIMM in each of its two slots. It must be the EXACT correct spec of that MacBook Pro model, and you should deal only with a Mac-Centric dealer who stands by their product. Otherwise, you can end up with modules that are flaky, and cause your Mac to crash several times a week.