How to modify the bag file?

edit

I planned to test the result of kinect signal after some information in points cloud is corrupt to simulate the bit error after wireless transmission.

So I firstly save these points could data by rosbag, and I can replay it to rviz project.

However, I then want to change certain bits in the bag file to see what happens when some points data is wrong, and I faced two questions :

1. bag file formate is different from what rosbag-format wiki told? I try to make a simple test at first : write the publisher always publish Hellow World xxx as the beginner tutorial did, and bag this topic.

   My result bag file looked like this : E^@^@^@^P^@^@^@chunk_count=^A^@^@^@^O^@^@^@conn_count=^A^@^@^@^R^@^@^@index_pos=^A&^@^@^@^@^@^@^D^@^@^@op=^C»^O^@^@^R^@^@^@^N^@^@^@Hello World592&^@^@^@ ^@^@^@conn=^@^@^@^@^D^@^@^@op=^B^M^@^@^@time=ahïNlN<9a>1^R^@^@^@^N^@^@^@Hello World593&^@^@^@ ^@^@^@conn=^@^@^@^@^D^@^@^@op=^B^M^@^@^@time=ahïN^Fß<91>7^R^@^@^@^N^@^@^@Hello World594&^

So, I think it should be a plaintext(readable) like something "<header><content><xxxx>..etc", but it is not QQ. How can I ask decrypt it to a readable plaintext?

1. After I change the the "Hellow" in the bag file to "ILoveYou", and replay it : My subscriber still get the Hello World ........not the ILoveYou World????

This is my first time to use ROS, I have read all overview and done all tutorial in the wiki... Could anyone give me some suggestion????