Is it a right thing to do?

Don't do it for her. If it's something specific and obvious like you're blonde and she likes dark guys, it'll only seem pathetic that you would drastically change yourself after she clearly said she wasn't interested... If it's more along the lines of you're a little overweight and you think she's into trimmer guys...then maybe you can start working out and getting a little toned and in shape. That will be a gradual change and one day she'll look up (or some other BETTER girl) and say damn, he's hot. Then you can reject her! What ever it is and ask yourself if it's a true improvement on yourself you want to make or is it clearly a shallow preference she may have in which case another girl will like you just the way you are and you should go find her.

no.. not necessarily.. it depends if the change is better for you anyways.. I mean yeh people will say you should do it cause you love yourself.. blablabla but being realistic.. I know how it is at our age.. etc.. whatever.. if its something that is better for you then its not wrong to do it.. you just happen to have more inspiration to now.. but again.. let me know what the physical change is.. cause I mean id like to be able to really give you advice appropriately and tell you what I mean by if its a good change for you..? when I say good.. I mean if you are overweight.. hitting the gym is a good change.. if you are a smoker.. quitting is a good change..?