Your question is too generic. It's better to give specific example or block usage.
A "parameter" of a block is meant to be constant during one whole simulation, for example, the "gain" value of a Gain block.
You can change the parameter before or after a simulation.
There are ways to generate a value dynamically and use the value dynamically in Simulink, but not to use the value as a "parameter" for a block.
There might be ways to change the implementation. For example, if the "gain" value needs to be changed dynamically based on some calculation, you can use a "Product" block to multiple the signal with the calculated value.