As is well known, the battle of the Teutoburg Forest was an important turning point in the history of the Roman empire.
Now a time traveller goes back into that time in order to change history, with the goal to have Varus win. However it is not possible for him to take any modern/future artefacts with him; not even the time machine itself. He can only transfer himself back in time, and program a point in time when the time machine automatically takes him back to his own time; if necessary, he can repeat that several times (provided his change in the time line didn't make the time machine vanish). So all he can do is to learn as much as possible about that time, and use that knowledge to try changing the history using contemporary means.
So what could he do to make Varus win? Of course he can inform him about the plans of Arminius, but then, why would Varus believe him? And could he do anything else? Maybe use future knowledge to provide Varus with better weapons?
Note that he can't simply kill Arminius as child, as that way Varus would not win the battle because there would not be any battle.