Fixed Width text
Fixed width file formats are 'padded' with spaces between the columns.
The fields are identified as a count of positions/characters from the start (leftmost) position on the line.

Press 'Next'.
Identify field breaks

The Source data (red) will be shown on the top grid and remain unchanged. The formatted data (green) on the bottom grid will be displayed with the field breaks.
Create a field break
Click your mouse in the Source (red) grid between the characters where you want the field break located.
A vertical line will be inserted to identify the field break in the Formatted Data grid (green).
Note: The source data file is never altered.
Before click After click

Tip: For best 'mouse response', avoid clicking on the horizontal lines separating the rows.
Delete a Field Break

Click your mouse in the field header in the formatted (green) grid.

The field break is removed.
Note: There is no 'move' function for field breaks. Simply delete the one in question and reinsert a new one.