I have not used fields for many years, so had to do some research

Probably the first problem is to actually identify which part of your document is actually a field, as opposed to normal text entries. To do this, click on 'Edit/Find' (shortcut Ctrl+F), and then GoTo. In the left hand box, called 'Go to what', scroll to 'Field'.
When you locate the right spot, the field should be highlighted... make sure your cursor is in the field, then just press Control F11. To unlock it, press Ctrl/Shift F11.
The help screens in Word are very helpful- if I have not explained it properly, just click Help, and type in 'Fields'.