Replace existing value with already populated input when the user navigates back to the cell

[devraj]

• User enters a character is navigated to the next cell
• User presses back space or touches an arbitrary cell to enter text
• The cell would already have a value in the field and the cell would restrict entry to a single character
• If the user enters an acceptable character the cell should replace the old character with the new one
• Note that if the user presses backspace it should function as expected