how do I update my OS on a MacBook pro 2008 to Catalina

You can't your mac cannot upgrade any further, El Capitan is the last OS your mac can install.

Only macs 2012 and newer can upgrade to Catalina.

This from Section 1 of the How to upgrade to Catalina guide,

Check compatibility

If you’re using one of these computers with OS X Mavericks or later,* you can install macOS Catalina. Your Mac also needs at least 4GB of memory and 12.5GB of available storage space, or up to 18.5GB of storage space when upgrading from OS X Yosemite or earlier.

MacBook introduced in 2015 or later

MacBook Air introduced in 2012 or later

MacBook Pro introduced in 2012 or later

Mac mini introduced in 2012 or later

iMac introduced in 2012 or later

iMac Pro (all models)

Mac Pro introduced in 2013 or later

* To upgrade from Lion or Mountain Lion, first upgrade to El Capitan, then upgrade to Catalina. To find your macOS version, Mac model, memory and storage space, choose About This Mac from the Apple menu . If your Mac isn’t compatible with macOS Catalina, the installer will let you know. View the complete list of compatible computers.