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The window film used to tint the windows on automobiles will deteriorate with time. The dying film used to create the darker color will begin to turn a purplish color or the adhesive used to hold it in place will begin to fail and bubble up. In either case, it's unsightly.
Bubbling window tint
The good news is that it's easy to remove, so long as you know how to do it. The first thing to know is that you should not just peel it off.

Window tint is made up of two layers: an adhesive layer and the tint layer. Without any treatments, peeling the tinting off leaves the adhesive behind. This means hours of scraping.

You can avoid hours of scraping in one of two ways:

The Easy Method: Steamer

If you have access to or are willing to spend $30 on a carpet steamer, you save a lot of time and effort removing your window tint. Just turn the steamer onto your window for a while. When it gets hot enough, the moisture will penetrate between the glue and the glass and cause the tinting to begin to fall away. Simply peel it off and use an adhesive remover for the residue.

More Complex, But Cheap Method: Garbage Bags

This method is the more common, but requires more time than the steamer and is messier.

Cut two black garbage bags to roughly fit the window - the easiest way to do this is to cut the bags open (unfold them), lay them over the window, and use scissors to go around the edge so the bag is about the same size/shape as the window. It doesn't need to be exact. Again, you will need two of these.
Spray soapy water on the outside of the glass and place a bag over it.
Smooth it (use a rag or glove) to the glass to push out as many bubbles as possible.
On the inside, cover everything under and near the window with a tarp or plastic sheet.
Using a spray bottle of ammonia, spray the inside of the glass to thoroughly soak it.
Then place the garbage bag over the inside of the glass and smooth as before.
Note: some people will have bad physical responses to ammonia or its fumes. Be careful with it and use only in a well-ventilated area.
Leave the vehicle in the sun. The black garbage bags will absorb sunlight and heat. The area between them will become very hot and the water and ammonia will steam. The soap will keep the plastic bag from melting into the window while the ammonia on the inside will break down the glue holding the tint on.
After the tint begins to peel, pull it off and use an adhesive remover to remove the last of the residue.
This method obviously works best on hot summer days and should be done when several hours of sunlight are available.