Is it?

 when [left arrow v] key pressed
switch costume to ((costume #) - (1)) 

Yes - I believe it is.

Yep. Switching backdrops is easier, for whatever reason, but this is it for costumes.

The add and subtract are done “modulo” the number of costumes. If you do -1 on the first costume it goes to the last one ;-)

There is an easier way yet it requires you knowing how many costumes are on the sprite.
If there are 7 costumes then make it change by 6.
Or if there is 5 costumes make it change by 4 ect.

KermitMC wrote:

There is an easier way yet it requires you knowing how many costumes are on the sprite.
If there are 7 costumes then make it change by 6.
Or if there is 5 costumes make it change by 4 ect.

The script would look like this.

when [left arrow v] key pressed
repeat [4]
next costume

I don't know, that doesn't really seem easier, and it's also not future proof. If you add a costume, you have to change the number.

Or, this:

switch costume to (join [costume]((costume #)-(1)))