Neither, you need to tackle this problem step by step.
You need to create a random vector of integers. The second example doesn't return integers, while the first does. Since the excersize didn't specify any maximum value 32 should be fine. So x=randi(32,32,1); can stay.
Now to the loop. We need to look at every 7th element. There are two basic ways we can do that. We can either have some value and multiply it by 7, but in this case the other strategy is better: use the two-colon syntax to generate list of positions to change. You did that in your second code block when creating z.
Inside the loop we need to change a value. That mean we need to overwrite the value:
%set some value to var
var=3;
%increment var by 5:
var=var+5;
The doc of the for keyword provides some help how we can continue here: look at how you can change the first example to change a specific position in a vector, and the second example for how you can enter your own list of indices.
Nothing in the assignment says anything about displaying anything, so disp and fprintf are not needed.
If you need any more help, just write a comment.
