Because 13 couldn't afford to restart the computer to calculate reentry, the crew were going to have to navigate the same way old sailing ships did: by looking at the stars. As long as they kept the top-left corner of the Earth in the front window, and the sun in the window directly above them, they would be on the right track. It wasn't a precise form of navigation, but it was the best they had.

–Epic Fails: The Race to Space: Countdown to Liftoff,
Erik Slader and Ben Thompson

How did the astronauts have to navigate the reentry?

They had to calculate the reentry by hand.
They had to use the computer to guide them.
They had to look at the sun and Earth through the windows.
They had to make sure they could be seen from ships on Earth.