Flex Builder error : Namespace 2.6 in the application descriptor file should be equal or higher than the minimum version 3.1 required by Flex SDK.

"Namespace 2.6 in the application descriptor file should be equal or higher than the minimum version 3.1 required by Flex SDK."

what does it mean ?

to run the application

1) i need lower version than 3.1

2) i need higher version than 3.1

i am confused.