Just manually push it back in.
But as always, check your hospitals policy for numbers on when to hold, how much to re-infuse, etc.
Also, when you have the time, if you have a shared governance or whatever, look up and then pass along the latest research on tube feedings. Current studies show less than 500ml is no issue and even not checking residuals in intubated patients. There are obviously exceptions to these rules, but it is what current research shows.
Also, post pyloric and J-tubes you wouldn't check residual...
Ultimately though, it doesn't change your practice until a policy is changed, hence, check your policy. A lot of times hospitals will also have access to resources that cover nursing tasks and outline the steps to do things (aka Lippincotts [sp?], etc). Sometimes the policy will just say do "this" and you have to refer to the proper procedure there. Just FYI. Great things to address with your preceptor or other experienced RN there.