First check to make sure it's not your blade. Are the cuts "wandering", or is the rail really out of square to the blade? Use a speed square to check.
If the blade is wandering, try a dressing stone on it. If it still won't cut straight, you probably need a new blade.
If the rail is out of square to the blade you can adjust it, bear in mind though, it's a little tricky and will take a while to get it properly adjusted. Better have some scrap tile on hand to check it multiple times.
There are machine screws on either end of the rail, if you have never adjusted them, you may need to hit 'em with WD-40 and let it soak in for a few minutes. You can loosen one, or both, of the screws and move the rail within the slots on the frame.
You can get a manual for that saw HERE. It doesn't explain how to adjust the rail (they want you to take it to a shop for that), but there is an exploded parts view where you can see exactly where the adjustment screws are.
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