It is too late to file any extension at this late date ... you have until 10/15/19 to efile the return after that date you must print & mail the return. Now if you have a refund nothing special needs to be done when filing the return.
However, if you owe then you should print the return and write your deployment dates & the combat zone across the top of the 1040 to indicate you were deployed and have an automatic 180 days to file the return once your boots hit home ground. Read up on military matters in the IRS Pub 3.