How do i change a slider during a simulink simulation?

Hi,

I'm a new user of matlab and i've got a question about Simulink. I want to simulate the outcoming flow of liquid from a tank.

I've got a tank with a volume of 30,000 litre. When i open the valve the outcoming flow is between 0 and 200 litre/minute. I want to change the outcoming flow during the simulation. I've added the example, i made in simulink. http://www.mathworks.nl/matlabcentral/fileexchange/47532-example-slx

I can't change the slider during the simulation. Can anybody help me with this problem?

Thanks.