It would require a change in the law, and possibly the Constitution, to do so.
The election date for federal elections is fixed in 2 USC 7. Congress would need to pass a law to change this date, and even then, it would only be good for a month or so.
Changing this law, however, does not change the fact that the Constitution sets out the end of a Congressional term (January 3) and Presidential term (January 20). Thus, in order to delay the election beyond mid-December, an amendment to the Constitution would be required to extend these terms.
It is worth noting that the US held elections in 1944, in the middle of World War II, as well as in 1864 during the Civil War.