What can I do to defrost my wiper fluid?

If there is room in the tank, add some winter fluid. While it will not actual melt the ice of the summer fluid, as the heat of your engine melts it, the winter fluid so keep it from being able to refreeze. Depending on how much you can add, you may need to empty and refill as it melts to keep the concentration of the winter fluid up.

Another alternative is heat water to melt, then empty, and refill with winter fluid

You could just sit there and direct the fan to blow against the glass and defrost the window. That will take awhile.  Or you get out of the car and scrape the window with a plastic credit card to get the frost off...or you own a snow scraper and get it off that way.  Wiper blades won't clear it and you cannot use the windshield washer anymore until you change it to not just water.  Buy windshield washer antifreeze.(it is not the same as radiator antifreeze) they are not interchangeable.  The windshield washer stuff may have soap in it and you do not want soap in a cooling system.

Wait...…………….. .

Park the vehicle in a heated garage for a day or take the wiper fluid tank out and thaw it in your house.

Run the engine to full operating temp.

Then either drain the washer fluid or add fluid that can handle winter.

Finally NEVER switch back to whatever you are calling 'summer wiper fluid'.

You'll have to bring the truck into a heated garage. Nothing else will work.

Dump some winter wiper fluid in there

i would get a mechanic to help you out with that

Go to a coin operated car wash. If the water is still flowing, hose off the windshield wipers and sheet metal below the windshield. Or find a heated garage and park over night.

The fastest thing would be to remove the fluid reservoir and put it in a sink of hot water.