To download data for a project, go to the project home page and click on "View Data":
On the navigation bar at the top, click on "download" to open a left side panel with the download settings:
A panel will slide in from the left:
Select the mapping you want to use (The default mapping is always pre-selected). More on mapping data
if you do not know what mapping is, just ignore it.
Pick JSON or CSV format.
Click on the project zip file to download it.
If you have more than one single form and branches, you will get separate files: one file per each form and one file per each branch. How to merge the data (if needed) il left to the user. For example, using Excel, there are several ways to merge tables based on common colums (in Epicollect5, look at ec5_uuid, ec5_parent_uuid for child forms and ec5_branch_owner_uuid for branch forms), have a look at this link.
An example using Google Sheet can be found here.