Hi everybody,

Can I change this "Look at the sign up there" to just "Look up there"? Two people are under a big sign (a timetable for busus?) and one of them doesn't seem to know which bus he have to buy the ticket for, so the other person is trying to tell him that there's a timetable for buses' schedule. Thanks!

Or you could just point and say 'Look'.

Or just point and say nothing.

"Look up there" is fine if it's clear what you want them to look at (as seems to be the case here).
 Clive's reply was promoted to an answer.