The Case Conversion button is available on the collection toolbar, except when the Recycle Bin tab is active.
When you click the button, a menu will appear where you can select the type of conversion you want to perform. You can also access the Convert menu directly in the main menu.
If you selected a part of the active clip's text, the conversion is applied to the selected part only. Otherwise, the conversion is applied to the entire text of the clip. If the clip has before and after text, the conversion is applied only to the edit box that has keyboard focus (i.e. the one in which you typed last).
See Editing Clips and Text to learn how to use AceText.