Try increasing the navigation meshes agent step height and slope values inside of your project settings.
Project settings → Navigation Mesh → Generation. You will have to play around with the settings to get your desired outcome, but the ones most useful for this would be “Agent Max Slope, and Agent Max Step Height”. You should see the green navmesh update and go up the stairs, indicating your AI will also be able to if your mesh is setup right.
Hope this helps!