how to improve hand writing ?

Grab a journal book and a pen and practice your letters like you were in kindergarten or first grade. Make an effort to form the letters properly, don't rush. I used to do that all the time, now my handwriting looks like a handwriting font and not what came out of the end of a pen. Lots of practice and patience. If you start seeing your letterforms go messy, stop and refocus.

I'm assuming it's your math and science classes that are hurting you. Try learning to use LaTeX if it's a really serious problem. It's a nice formatting language that can produce easily readable math notation.

Basically just type everything that you can.