Complex sentence: It is better to tweak it according to new content that has been added to the article, or if one finds that very significant matter in the article is not mentioned in the lead, then add short mention in the lead.
Split into simpler sentences: It is better to tweak it according to new content that has been added to the article. If one finds that very significant matter in the article is not mentioned in the lead, then add short mention in the lead.