en.javascript.info/2-ui/4-forms-controls/2-focus-blur/4-edit-td-click/solution.md

1. On click -- replace innerHTML of the cell by <textarea> with same sizes and no border. Can use JavaScript or CSS to set the right size.
2. Set textarea.value to td.innerHTML.
3. Focus on the textarea.
4. Show buttons OK/CANCEL under the cell, handle clicks on them.