Hello Dafnis14 -
They would open the app and go into the Data Load Editor and click the Load Data button.
Providing the Folder connection path is pointing to the same location where the file is located, they would not need to change this.
BTW - If you have not seen this already - there is a complete list of comprehensive videos that will show this process when using apps with Qlik Sense.
Please mark the appropriate replies as helpful / correct so our team and other members know that your question(s) has been answered to your satisfaction.
In the Qlik Sense full platform (server), reloads are done unattended on a scheduled basis or as needed.
The Qlik Sense full platform offers a full set of APIs so something like you have mentioned (button) can be done programmatically from within an app mash-up. (Think of it like a web page with the app layout - running in a web browser with a button to load the data on demand from the source)
However, just for your reference, Qlik Sense Desktop is designed for FREE self-service local desktop analytics and data visualization, it is not designed to be deployed in the manner you are describing.