If your text is upside down and backwards, you messed up. Hold it up so you can read it again, rotate it 180 degrees, and drop it into the tray. Repeat for the rest of the slides, then put the ring back on snuggly. Get an inkpad that you use for rubber stamps and a new pencil with an eraser. Then put an ink dot with the pencil eraser in the bottom left corner of each slide mount.
T hat's an unusual situation, but it's fairly common.
Essentially the projector is hitting the screen at an angle, which produces the focusing problem. If the slide projector is lower than the screen and it shoots up slightly at the screen, then you have the same effect. Only the focusing problem with be a top to bottom thing. If it's off by a few degrees, then it will cause the keystoning focus problem. T o solve it, you need to have the slide projector as high as the center of the screen.