how can i update to catalina os

Is your MacBook Air a 2012 or later model? If it is, then you can upgrade to Catalina if you have sufficient remaining drive storage (> 25 GB). Check the Mac product description line from menu : About This Mac panel.

View the following link in Safari only:

How to upgrade to macOS Catalina

Understand that Catalina can only run 64-bit applications, so don't upgrade if you are dependent on these, among:

• Microsoft Office 2011 for Mac
• Adobe CS6 or earlier
• Java 6
• Apple iWork '08, '09 suites
• Apple iMovie < 10.1
• Apple iPhoto
• Apple Aperture
• Print Shop 4 or earlier
• Printer or scanner drivers for earlier operating systems that remain 32-bit

Do not upgrade without a final Time Machine backup of the current operating system. Do not use that same Time Machine drive to backup Catalina.

You need an external Hard drive then...

The free, built-in solution: Time Machine.

Carbon Copy Cloner...

http://www.bombich.com/software/ccc.html

Give Tri-Backup a try, it can do everything & far more than TM can do...

https://www.tri-edre.com/english/tribackup.html 

Hi, welcome to the Apple Users Community.

This link will help let you know if it is possible to do and show you how.

https://support.apple.com/en-us/HT201475

Best of luck