This chapter of the help file shows you all information about a Preamble in the GraPE-specification. A Preamble always exists in a Process Diagram. There is only one action that can be done with a Preamble and that is change the Preamble.
A Preamble is visualized as a purple rectangle. The figure below shows the Preamble.
This describes how you want to change a Preamble.