Any inserted text should be marked with <ins>...</ins>, which renders in most browsers as underlined text, e.g., inserted. Best practice is to add a new timestamp, e.g., ; edited ~~~~~, using five tildes, after the original timestamp at the end of your post. To add an explanation of your change, you may add a new comment immediately below your original or elsewhere in discussion as may be most appropriate, insert a comment in square brackets, e.g., "the default width is 100px 120px [the default changed last month]", or use [[WP:CURRENTSECTION#New section|<sup>[corrected]</sup>]] to insert a superscript note, e.g., [corrected], linking to a later subsection for a detailed explanation.