Add the “reformat” column to your bin. Hilight all clips that you want to adjust and right click in that column. It is a menu with different aspect ratio controls. Pick the one that you want.
For sequences that already have these clips included, with sequence/timeline active in record side window, go to clip menu (and I’m trying to remember menu item off the top of my head), select sequence, update sequence or something to that effect and update aspect ration from secondary menu.
Michael