Basically, this is how it should work:
You tell the program to find one or more specific tiles.
Then you tell it to replace the previous tile(s) with one or more different tiles.
The tiles found will be randomly replaced around the level.

Example:

This is what you got in the level:


You tell the program to find this:

And replace it with these:


Then you do the same with this:

And you replace it with these:


The program will give you something like this: