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.