How can an app use files inside the JAR for read and write?

I need to store data into files inside .jar file and read it again

No you don't.

Instead store the 'default' file inside the Jar. If it is changed, store the altered file in another place. One common place is a sub-directory of user.home. When checking for the file, first check the existence of an altered file on the file system, and if it does not exist, load the default file.

Note that it is generally better to describe the goal, rather than the strategy. 'Store changed file in Jar' is a strategy, whereas 'Save preferences between runs' might be the goal.

Related: What is the XY problem?

This is not really supported. In principle, you could operate on the jar file, but there's no guarantee the new contents would be correctly loaded. Let your build tools manage the jar file -- and choose something else for persistent storage managed by your program itself. Like a file.

It is unlikely that you can change a loaded jar safely. Changing it while it is being used is not a good idea.

If you want to do this, use a plain file system directory and add/remove files from it. Even this may not work as you expect.