In previous moodle mobile version, there was an option to change the siteurl using the config.json file. But it's not working for version 3.5.
Moodle for mobile
How to set Prefix URL for moodle mobile 3.5
it does still work, but you need to "compile" the code. In the previous app it was done using "gulp", now Ionic has it's own build system. Just run "ionic serve" or compile in AOT mode.