Click to watch the set costume num tutorial
Click the block button
Get the set costume num block
Drag the set costume num block into when sprite1 collides with coin class.
Drag the random block
Drag random block to set costume num block
Click the field that says "sprite1" and select "collidee2"
Click the second field that says '10' and change to "4"
The first costume is chosen on a result of "1", the second costume on a result of a "2", etc.
Click the play button
Now, when you hit a coin, it will change to a random costume available to its class.