A low pass filter has the transfer function G(s) = 1/(Ts+1). Typically you would use a Transfer Function block to model this in PLECS. To be able to control the time constant T with an input signal, you need to implement the transfer function with discrete Sum, Product and Integrator blocks.

You have: G(s) = Y(s)/U(s) = 1/(Ts+1)

⇒ Y(s)*(Ts+1) = U(s)

⇒ ...

⇒ Y(s) = 1/T * 1/s * (U(s)-Y(s))

This is implemented in the attached model.