Hi there
If you go settings -> general settings -> AnkiWeb account, you will see a log out button
However, if you log out and log in as another AnkiWeb profile, it will wipe out the old profile information (make sure you have synced first!), and the same will happen if you switch back, since AnkiDroid does not natively support profiles.
If you really have more than one AnkiWeb account and you want to use both of them for different reasons (like I do), then you will want to check this out:
Which has a pointer to the releases section on my AnkiDroid fork where I put parallel builds up of recent versions. I use them myself so I can have a collection for my actual learning, a collection for my son, and a couple collections for testing.
Hope this helps -
-Mike