Question

How would the seasons change if the earth were tilted at 90 degrees instead of 23.5?

Answer 1

The 23.5 degree tilt is responsible for the seasons. If the earth had no tilt there would not be seasons. If the earth was tilted by 90 degrees the seasonal changes would be at the most extreme. The Earth's pole would point directly at the sun at a point on the track around the sun. As the Earth revolves around the Sun the pole would alternate twice each year between pointing directly at the sun and being perpendicular to the sun. 

I hope this helps you! 
xo, Leafling