Once you've got something selected it's a simple matter of seeing what actions are available. These actions are highlighted in the different tabs of the ribbon.
Examples of available actions
Available actions in the tab "Edit":
Available actions in the tab "Notes":
Available actions in the tab "Home":
Applying an Action
Simply click on one action in the ribbon menu to take effect on your selected element or your score. F.e.: You will find the "Transpose by key" action in the main tab "Home" in the group "Transpose". If you have selected only one note there will be only the action "transpose by interval" available. To select a whole track just double click on the staff. Then you will be able to define a key only for that selected track (transposed instruments). If you don't select anything you can change the transposition of the whole score.
You can use the dropdown list in the Properties Palette as an alternative to selecting an item from the Actions. In either case, the appropriate controls are displayed in the Properties Palette for the chosen action.
Once you have configured the controls for an action, simply press the Apply button to apply the action to the current selection.
One of the easiest ways to try out an action is to select a loop region and play it while applying different actions. Most actions can be undone, so you can change your mind later.