Simply create a new sequence with the proper settings.
Copy paste the clips from the incorrect sequence into the new correctly set up sequence.
The clips might not fit properly at first. Double click one clip in the sequence to open it in the Viewer window. You made need to adjust the Scale and Distort perimeters. Scale should be 100 and aspect ratio in the Distort section should be 33.33 (or maybe it's -33.33)
Once you fix one clip, highlight it in the timeline, right click, copy. Highlight the rest of the clips in the timeline, right click, Paste Attribute. Be sure to select scale and Distort. Then everything should fit nicely.