Itīs part of the process opening an airline,it also can be done later. Just take a tour through your settings.
If you want something different than given options, you are free to design one on your own.Just control the given formats as well as size restrictions before importing to Airway Sim. If you think something more than a full livery for your fleet, thereīs a livery design request thread in General Forum, if you canīt design one on your own. There are templates for various aircraft models free to use across internet besides the ones that airway sim has in stock, just google them, if you wish to do your own design.