How can I make a loop for sending a signal in form of step, using only Matlab and Arduino Mega 2560 (not Simulink nor step function itself)?

Hi everyone

I'm working with arduino Mega 2560 and Matlab and I'm using the function "writePWMVoltage" for sending a specific voltage to outside.

My issue is the following situation:

I don't know how to make a loop for creating a "step" signal (without using the function of matlab itself, nor Simulink).

For example, I want to send a voltage of 2 volts since t = 0 to 30 seconds, and from the instant 31 second, send a voltage of 2.5 Volts (If you see, this is like a "step function")

I'm trying with the functions "for", "while", and I don't know how to get this thing, it appears some errors, I'm a amateur in programation.

Please help me.

Thank you so much for reading and answer me in advance.