Tiene que descargar esta aplicación ahora mismo.

English Translation

You need to download this application right now.

If this is “you need to” shouldn’t this be “tienes” and not “tiene”?

English “you” corresponds to , vos, usted, vosotros, vosotras, or ustedes, even impersonal se. Even if the subject is not explicitly stated, the verb form varies accordingly. This is directed to usted, and thus uses the third person singular.