Hola Belinda
When we use the formal pronoun "usted" instead of the informal "tú" we need to conjugate the verb, in this case "estar" as in the 3rd person singular, the same way as with he/she:
Tú estás aquí (you are here [informal you])
but
Usted está aquí (you are here [formal you])
Él está aquí (he is here)
Ella está aquí (she is here)
Hope this helps,
Saludos
Inma
