macOS version update: software update from 10.14.6 to 10.15 is not available on my MacBookair

Hi Apple folks,

I'm having some issue trying to install ''Numbers'' on my MacBook air due to the following message:

<< Numbers can’t be installed on “Macintosh HD” because macOS version 10.15 or later is required >>

Could you please help me to update/upgrade my macOS Mojave version 10.14.6 to 10.15 or later?

Thank you

Hamidouch