If I understand your question correctly, then you need to make a condition and by condition (the analogue "if" in other programming languages in Simulink is the Switch block), change your data. The condition will identify a specific sequence or value, and mathematics after the condition will change the value. For example, if you have a value of "1" and the condition identified it, and you need to change it to "5", then after the Switch block, you multiply "1" by "5" or make another Switch and when the first is triggered, the second one connects to its output value of the desired constant.