Can I edit the contents of a cell (and not overwrite it)?

Yes. Follow the standard spreadsheet (e.g., Microsoft Excel) conventions; specifically:

Double-click on the cell. To avoid clearing the contents of the cell at the point when you start entering a correction, double-click on the cell before you start typing. This action enters the edit mode and positions the cursor within the cell.

Press F2. Press the F2 key on your keyboard, which enters the edit mode for the currently highlighted cell.

ToolTips for Cells. When placing your mouse pointer over a cell (hovering) whose content is not completely displayed, a ToolTip containing the full contents of the spreadsheet cell is displayed. When hovering over a text label, the ToolTip also displays the label's numeric value. When hovering over a cell containing a numeric value displayed as #### (because the column is too narrow), the ToolTip displays the full value of the cell using the current variable's display format. ToolTips are also displayed in the Variable and Case header cells if their contents are completely displayed.

See also, Range of Numeric Values that Can Be Entered or Stored in Cells.