"stem Up" as well as 
"stem Down" you will find in the toolbar category < page >.Change Stem Direction Or "Flip Stems"
If you want to change the direction only for one individual note, so you use following possibility:
The button (Icon) "stem Up" as well as "stem Down" you will find in the toolbar category < page >.
In order to alter the stem direction for a beamed group of notes, the first note of the group must be marked at least.
Mark the group of notes which stem-direction should alter, and then click on as well as.
(shortcuts "u" = up, "o" = down)
To change the stem direction for whole voices use the Main-line edit dialogue Tab (page) "Stem direction.