Views and HTML Modifications
This type of customisation is just for experts. Editing the source code of a specific page of the component front-end is probably the most complex operation that you could do on an extension, beside (obviously) altering the PHP code. If you need to modify the PHP code of a Controller, a Model or a View Declaration, then there is no solution. This would be a core modification and no one would help you save those modifications when installing an update.
However, if you just need to edit the source code of a specific front-end View, there is still a solution to keep the modifications even after installing an update. It's again a native function of the CMS Joomla so why shouldn't you consider it? This technique is called 'Template Override'.
Let's say that you would like to change the HTML structure of the View 'Rooms List' of VikBooking. The first step is to locate the file 'default.php' that generates the output of that page. In this example, the file will be the following:
All the Views have the same structure in a component for Joomla designed according to the MVC standards. The file you are interested into is the 'default.php'. In order to create an 'override' of that View, it is necessary to copy that file onto the following directory:
Some folders in the path above may not exist because your Template may not have an override for VikBooking. You should create the missing directories in order to have the file 'default.php' in that directory (YOURS stands for the name of the folder where your Template is installed).
In this example, if that file existed, Joomla would read that override rather than the component original file. That file could then be modified without any fears of being unable to install future updates.