Step 1: File at location:
contiki/core/dev/cc2420.h declared transmission range.By modifying this we can change ranges.
// parameter power Between 1 and 31
void cc2420_set_txpower(uint8_t power);
int cc2420_get_txpower(void);
#define CC2420_TXPOWER_MAX 31
#define CC2420_TXPOWER_MIN 0

Step 2: Another way to modify
A file with (.csc) extension cooja file will be created after compilation in cooja.
Open a file and change the transmission range.

Step 3:right click on node >> change transmission ranges .