In the Airtech theme, by default Gallery is being used to display the Services.
So to edit the Images of the Gallery, please go to the Service menu in wp-admin and edit the Featured image of the given Service item. These are the images used in Gallery.
If you prefer stand-alone Gallery feature, it’s recommended to use a plugin to better manage your Images and Gallery, we suggest trying this plugin https://wordpress.org/plugins/nextgen-gallery/ however, there are many awesome plugins available for Gallery in WordPress. They can be found with a simple Google search.