Let's have a look at the concept of the AM modulation.
The simplest linear amplitude modulation formula (for DSB-SC) is:
m(t) = A(t) \cos(wt + \phi).
In amplitude modulation; you have a carrier signal that has a frequency that is fixed, $\phi$ function can be omitted, and a transmission signal which has it's information in $A(t)$ function. So we are basically modulating our signal to shape of the carrier.
There are different techniques for demodulation of AM modulated signals. The most basic method we can think about how to demodulate this signal is basically detecting the envelope of the signal. In real world, it is a matter of a diode and a well decided capacitor. In MATLAB, you have to simulate those characteristics.
There is an example of envelope detection here.
There is another way which is waaaay more easier to implement in MATLAB. The method is called "phase locked loop" (or something like that). Note that you can use this method IF your signal IS synchronous. The block diagram of this demodulation method is like this:
The most common mistake when applying analog demodulation techniques in MATLAB is sampling. You have to take your samples on the right frequency, and at the right points.
I hope this is a bit enlightening for you in the terms of amplitude modulation techniques. There is more of the demodulation methods, however who needs all of them anyway? Just take a look at them and find the method that fits for your needs. I used phase locked loop the most so if there is no specific needs, I suggest you to use it.
As always, Have a nice day.