Assuming you mean editing? Press F2

You don't mention the version your using but in Excel 2003 a search
in the help file on keyboard shortcuts will get you all combinations.

gls858

Unless there is already something in the cell you want to edit, simply click
it once and start typing. Excel should automatically enter editing mode with
the currently selected cell.

If there is already data or text inside the cell, however, editing it in
this way will erase ALL EXISTING DATA IN THE CELL. This shortcut is best used
for entering data in new cells previously unused.