Extending validity is different for each time frame. Here is the USSD method of extension:
For 365 days It will cost 2 BHT per month
Dial *113*30*9# for 30 days extension in 2 BHT
Dial *113*60*9# for 60 days extension in 4 BHT
Dial *113*90*9# for 90 days extension in 6 BHT
Dial *113*180*9# for 180 days extension in 12 BHT