Write about how first impressions of people can change once you get to know them better.

I can't write this for you, but I can help by giving you some possible points to use for writing on this topic. 

-First impressions are pretty firm starting points for what you think about a person
-Think about a friend you have, and what you think about them now vs. what you thought about them when you first met. 
-The biggest change in people's personality is actually how you see them, not them changing. For instance, you might only see a specific part of their character when you first meet since they are not completely comfortable in a stranger's presence when they first meet someone.
-Think about how you act when you first meet people - are you open in your actions towards them, or completely comfortable being who you are? Or is it more likely that you are a bit reserved before opening up?
-It is likely that a big part of people's perceptions of others is based on how comfortable the other people are in being themselves with them.

Hopefully these are a good starting point. Good luck.