The most simple way to use SPCHR is with two arguments: the sprite's management number (you can think of this as the ID of the sprite, you set this using the first argument in SPCHR) and the definition number (the sprite you want to change it to). I'm not sure if I'm allowed to link the online resource to Miiverse, but search "Instruction List - SmileBasic" and search for SPCHR.
