How to change the slider while running the 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.

There is 30,000 litre inside the tank. The flow is a variable (0 to 200), that change in time. I want to change the variable (with a slider) to change the outcoming flow. I want to simulate this, till the tank is empty.

I tried to build it myself, but i can't change the slider while it's running.

How can is simulate this in matlab/simulink? Can anybody help me with this problem?