Android: Manually alter the contents of an app's database? Is it possible to manually change the contents of the databases an app uses to store all it's formation.
yes of course, so long as you have access to that database.

if we are talking about mobile phone apps, then most will have an embedded SQLite database or equivalent. These typically are "stand alone" databases that don't have user authentication to access, you simply need access to the file ("someapp.db", or "someapp.sqlite", etc) and the proper utility installed to read it (SQLite version 3 for example).

on linux you would do something like

sudo yum install sqlite -y

sqlite3 someapp.db

and it will open the database and you can issue all your SQL commands from there.

Keep in mind some apps go through lengths to "hide" or otherwise make their database impossible for average-joe to access and get into. 

EDIT: also, on Android specifically there is an App in the market (paid, like $3 i think) called "SQLite Editor" that allows you to browse all found sqlite databases on the phone and edit them if desired.