First, make sure that the situation is a war-time scenario: hard deadline (they rarely are) and shipping something (that enables sales, financing round or collecting customer feedback) is more important than shipping the right product.
In the war-time scenario focus is the key: reduce scope and ship only the bare minimum. This is as much managing up as managing down.
Get to know the team quickly, understand each members strengths and weaknesses. You likely need them all to succeed.
Daily task management and follow-up is essential. In the war-time you lead by asking questions and making orders. Prefer former, do latter when needed. Many (but not all) people like structure when under stress, so set it up if it doesn’t exist, but don’t let it restrict them to perform. Get people excited about shipping: even if you know the product is not great when the team will ship it, try to have a few shining parts there that everyone can be proud of, even if there are tons of ugly parts.