As Lucretia Borgia said this should help
(and for both iPhones and iPads)
Go to My Controls (under your username on the left of the screen)
Under the Menu (second column on the left) look for the Options subheading and click on Board Settings.
In the Control Panel, look for this bit ...
"Type of text editor to use when posting"
If it is set to Rich Text Editor, change it to Standard Editor (or vice versa).
Let me know if it doesn't