Let me make this statement upfront and be clear. The best way to solve problems with source data are to go back to the source and correct the problems there. This is my recommendation on how you should solve such issues. However, sometimes that is not possible for whatever reason. This article will explain how you can use Power Query to override incorrect data during load when you can’t change it at the source, for whatever reason.
This is the classic BI tool quandry: the best solution is, as Matt mentions, to fix the source system. But when that’s not on the table—such as when you’re getting data from a third party—Matt has methods to work through data issues.