Here's a parts explosion
The hose #9 you could drain through.
I like the coolant idea.
pour coolant in the reservoir, it might take a bit but it will turn the slush into liquid. There a glycerin in it and it's alcohol based so it's miscible with any alcohol in your wiper fluid. Nothing bad will happen.
Others chime in if this is ridiculous to you for some reason
Once its liquid, pull the hose #9 off the off the bottom of the tank and drain it. I can't tell from here but if the little pump is there you would need to operate it from the cabin to get the tank empty.
Once empty rinse it out good with bottle water.
reattach hose and then fill it up with non-freezing washer fluid from the convenience store.
Hope this helps!