• Open the clothing in S3PE
• If this is the first time you've used S3PE, tick the Names and Tags boxes at the bottom of the window
• Select the resource with the tag CASP
• Click Grid
• Expand the ClothingCategory line in the Grid window
• Change categories to True or False as needed
• Commit
• Save

Obviously, this assumes that you're editing CC clothing. If you want to recategorise EA clothing, you'll need to clone it first.

• Open S3OC
• Go Cloning -> CAS Parts
• Select the outfit you want to recategorise
• Click Clone/Fix
• Click Start and save the file

If you do this, remember to change the existing categories to False in S3PE, otherwise the outfit will show up twice in those categories.