My project is " How does amount of vitamin C change when fruits get older ?".
Please suggest me the process for this project.

Is this a LABORATORY PROJECT?
The Community College where I teach has a Biochem for Nurses Lab on determining the vitamin C content of cabbage. This would probably work on ripening fruit. Talk to your teacher to see if a nearby college or community college has a similar experiment that you could follow and to see if your school has access to the required reagents.