If you added a new column to the flat file, you need to update the Flat File Connection Manager
to reflect the new changes. Flat File Connection Manager will be present under the Connection Manager
tab at the bottom of the package.
Sample scenario illustrated using SSIS 2012:
Let's assume that you have a flat file with columns StateCode and StateName.

When you configure the Flat File Connection Manager
, you will see these columns configured under Advanced tab page as shown below.

If you modify the flat file to add an additional column, say by adding the new column named CountryCode.

The flat file connection manager will not contain the new column definition. You need to open the Flat File Connection Manager to add the new column or you could delete the Flat File Connection Manager and create a new one with the new flat file column definition.
You need to click New and select appropriate option to insert the column. You cannot move the column positions. So, make sure you select the right option to add the columns. Set the appropriate properties to define the column.

When you modify source or destination schema, it will affect the source and destination components within data flow task. You might see a warning icon on the component as shown below because the component is out of sync with the metadata information of the connection manager that it is associated with.
Double-click the component showing the warning and click OK on the editor to resolve the mapping issue.

Hope that helps.