Q: how do i sign out of macbook air?

Hi dfair59,

Welcome to the Apple Support Communities!

I understand that you would like to sign out of Messages on your MacBook Air. The attached article shows how you can access your account under Messages > Preferences > Accounts on your computer so that you can sign out. Please let me know if you need further assistance. 

iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support

Best regards,

Joe