Data rarely arrives ready for a report. Power Query is the part of Power BI that gets data from files and databases and cleans it before it reaches the model. Open it with Home > Get data or Transform data.
The editor has four parts. The Queries pane on the left lists every table you're shaping. The data preview in the middle shows the rows. The formula bar shows the code of the selected step. Query Settings on the right lists the Applied Steps.
Every change you make, such as removing a column or setting a type, is recorded as a step. Steps run in order each time the data refreshes, so next month's file gets the same cleaning without any extra work. Select a step to see the data as it was at that point.
Nothing reaches the report until you select Close & Apply.