How to correct the mistakes written by the ancients?

It is the easiest way to wipe it off directly with a brush. This is how Wang Xizhi and Yan Zhenqing handled typos. Therefore, Wang Xizhi's Preface to the Lanting Pavilion and Yan Zhenqing's Sacrifice to My Nephew can be regarded as "graffiti works" by two great calligraphers. The most common method is to add some dots with a brush next to the typo (usually on the upper right) and keep the original word to continue writing. Another way is to paint a mineral called "orpiment" on the typo, which is somewhat similar to the current "correction fluid" ... that's how the loose orpiment came from.