There should be a rotating selection menu available for the selected engine part under Advanced Part Settings or the Tinker Panel, in which the player can scroll left and right counting through the stages to change which stage number the engine fires on. Looks like the below, where # is the stage number:
< # >
This is helpful when the previous stages have 30+ engines and the player is adding a new stage, but unfortunately the engine part doesn't place itself into the correct stage number, and the player now has to search 30+ items in the list to correct the staging. Instead, the player can select the new engine and move it directly to the correct stage.