FMC-COMM --> DESC FORECAST

Hello,I have a question regarding transition level and altitude. As you know, not all countries have the same levels. I want to enter another transition level for my descent phase. I press FMC-COMM and select the descent forecast page to enter the descent winds. On the upper corner of the page it says Transition Level FL180. I try to enter a new value but this is not allowed.Where can I enter and updated transition level in the 747-400?Xander