How to reinstall a clean version of an app after deleting it?

I downloaded an app at my bosses suggestion. I used it, but I logged in with his credentials.

I deleted the app so I could reinstall a clean version to set up my own account on the app.

When I find the app in the store, it does not say "Get", instead it shows me the "Download from the cloud" icon. If I download from the cloud, it still has my boss' credentials.

How do I totally delete an app so I can reinstall a new "clean" version?

[Re-Titled by Moderator]

Original Title: App Clean Install