Go to the backend, in that select “Setting” from “Hotel Reservation System” tab. Then you will be redirected to "Hotel General Configuration" page. In that page, scroll down to configuration, you will find Hotel Background Image, in that click on add file and select the image which you want to display.
Using the changes in the link will change the view, but you will need to make changes that are for image panel only, do not make changes that are not relevant to images panel.
The upgrade module will not clean the database, all the data created such as rooms, images, pricing, facilities, etc. will be saved.
The upgrade module will only discard the changes done in the code and add all the new features.
once you install the QloApps it creates a fontend as to provide a fully functioning hotel reservation system. To edit the frontend you can create a new theme or edit the default theme "hotel-reservation-theme"
QloApps is based on Prestashop, please follow their doc to understand how theme works.