How to reset the panid of tag using gateway to let the tag join another network

hi may i know : now there is one tag connect to one network whose panid is 0x1111, then it move almost outside of network 0x1111 and in the range of 0x2222, may i know how could i update the panid of this tag from 0x1111 to 0x2222 so that the tag could join in the network of 0x2222.