What to do if my transmission fluid is overfilled?

Read your owners manual on just how to check the level. It varies from one make to another. If you check it correctly and find out it is indeed overfull take it back to the idiot who overfilled it and tell them to correct the problem.

I would have checked the level at the Valvoline shop before I took the car. It's kind of late now. I'd get some plastic tubing and a big irrigation syringe from the medical supply store and remove it through the dipstick tube.

I used to have an old fashioned sucker gadget (I bought it in the 1970s) that would suck about 16 ounces at a time, for changing oil in boats with inboard engines, but I'm not sure who sells something like today.

Whenever I did an automatic transmission fluid change in my own car or customer's cars, I always installed a drain plug in the transmission pan while I had it off to change the filter. Then draining ATF became easy.

-Engine overhaul mechanic and general automotive mechanic since 1972

I would just return there and ask them to remove some. They're the ones who put too much in.

make them change it again and get the level correct this time

What I use is a "Mr Goodpump", that I got at the part store, to put a rubber hose down the dipstick tube to suck out excess ATF. I saw one at Ace Hardware recently. It looks like this:https://www.finditparts.com/products/610184/dorman...

Add some blinker fluid to dissolve the access transmission fluid

If your transmission fluid is over filled the best thing to do is get it fixed.

Let your car warm up pull the stick out wipe it off and put it back in and pull it out and check it if it far over the mark. You need to go back to the transmission place that worked on it. And document on paper that they overfill your transmission so that if it is damaged they have to pay for it with their insurance. Yes there is a hand pump that you can put down the filler neck and suck out some fluid. But I wouldn't be doing that I'd be going back do the place that worked on it and get it documented in case your transmission gets damaged they have to pay for it.

There SHOULD be a drain plug on the transmission pan UNDER the car. If not, you'll have to loosen the pan until some starts leaking out. OR, SOME quick-change oil change places have a 'suction' machine to suck the old motor oil out of you engine before an oil change, that MIGHT work.