Editing

Open your score, exit from the Entry Note mode;
select a note and move it with the arrow up/down.

http://musescore.org/en/handbook/undo-and-redo
http://musescore.org/en/handbook/accidental

Depends on the type of change you want to make. If you are in note entry mode when you decide you want to make changes, first step is indeed usually to exit that mode. Then if you wish to enter new notes elsewhere, just click where you want to enter them and re-enter Note Entry mode, as listed in the very first step under Note entry in the Handbook. Or if you wish to make other types of changes, see the Handbook section for the type of element you wish to change. Or ask more specifically here if you are having trouble finding the appropriate section.

When I open a midi file in MuseScore, each part comes in with its own staff line --SATB.
Is there a way to get these all into one staff with two clefs like hymn format?

I think I posted this question in the wrong place. Feel free to move it.