In a flexible container, when the temperature of a gas increases, the volume of the gas will increase

PV=nRT
p=constant( because the container is flexible)
n=constant
R=constant
T increases=>V increases

OR

By increasing the temperature , you add energy to the system.By doing this you increase the speed of the gas's molecules , so the resulting atraction force among the molecules will be smaller , therefore it occupies a bigger volume.