Step 1.

Select cell

The cell border is dark and wide. The Status bar shows the word Ready.
Cell is empty: Formula bar is blank.
Cell is not empty: The Formula bar shows the cell's contents or formula.

Step 2.

Type text and/or numbers.

The Status bar changes to Enter. Characters you type appear in the cell, overwriting any previous contents. Navigation keys -Arrows, home, end- now move the cursor within the cell.

Step 3.

Press ENTER or
click the Enter button on the Formula bar.

What you typed is entered into cell. The selection moves to the cell below when ENTER key is pressed (by default), but not when is clicked.

Step 1.

Double-click cell or
select cell and then click in the Formula bar.

Cell already has content: The Status bar shows the word Edit. The cell border is dark and wide. The Formula bar shows the cell's contents.

Step 2.

Edit cell contents.

Arrow keys, HOME, and END now move cursor inside the cell. You can select a single word by double-clicking it. You can drag to select phrases or characters. You can insert new characters. Delete and Backspace will apply to the cell contents. Insert a line break with ATL + ENTER.

Step 3.

Press ENTER or
click the Enter button on the Formula bar.

Revised content is entered into cell. Selected cell is changed to cell below when ENTER is pressed (by default), but the selection is not changed when is clicked.

After editing and Before pressing ENTER —

Press ESC key.
or
Click the Cancel Formula button on the Formula bar.
or
Click the Undo button.

The cell's contents are deleted but the cell itself is not deleted. The formatting applied to the cell remains and will be applied to any new cell contents.

After pressing ENTER —

Select the cell and...
Press DELETE.
or
Click on the Home ribbon tab the button Clear and then Clear Contents... 
or
Use right click menu Clear Contents... .