If you use Blender, it's better to have backface culling enabled so that you have a clearer indication of how your model will look in game
select the affected faces in edit mode, left toolbar > shading > recalculate normals
Alternatively, you can select all the faces with A and press Ctrl N to recalculate the normals though hotkeys
If there's still inverted faces, you'll have to manually select them, press W for the Specials menu, then press F to flip the normals of the selected faces
