1) Have you tried to debug your code?

2) Why don't you synchronize your character reading with the framing characters of the RFID tags?

3) Have the cat eaten your comments? Is the reader expected to guess what baudrate you intended with:

#define Baud_rate 0xF4
 

4) Don't you believe in making sure that important variables not just have an initial value the first time you run through a loop, but are also correctly updated (set or reset) to prepare it for a second iteration?

5) You ask: "How can I modify [...]". Where is the information what you have attempted to do? What ideas you have considered? What you have tried? Where you have gotten stuck?