This photo of Dakota was taken when she was about 12 weeks old, and you can see that in this case her right ear is coming loose.
To take the ears down, you will need scissors (preferably bandage scissors) and a good gentle adhesive remover like Uni-solve.
Use as much adhesive remover as you need, and start with the tape that goes over the tip of the ear and down the back. Wipe up from the end, and the tape should just roll up. Keep rolling it up until the ear tip is completely free. Repeat with the other ear, I like to be sure both tips are free before I remove anything else.
Then cut the tape at the base of the ear up the back side. You can cut the bridge at this time too. Continue to wipe with the adhesive remover from the cut edges of the tape and along the edges of the stay. Don't pull - let the adhesive remover doH the work.
Once all the supports and tape are off the ears, clean the ears and examine them for any signs of soreness or abrasions. If there are any sore places, you'll need to protect them when you put the ears back up with a TINY bit of padding or by avoiding those areas by the use of a different sized roller.
Remove the excess tape from the stays and rollers, and put them in a safe place - dogs like to chew on them!
How long to leave them down?
That depends - if the puppy is still very young (less than 9 weeks) it's fine to leave them down for a day or so. The ear cartilage doesn't really start to harden until about 12 weeks. So especially if the puppy has a sore place on the ear, you can treat it and let it heal for a day or two. Ears usually heal very quickly if they do get an abrasion. And of course, if you do see a sore place on the ear, you need to figure out how to protect it by either padding it, or wrapping the ear in such a way that it doesn't cover the sore.
As the puppy gets older, you will want to put the ears back up pretty quickly. Every day you leave them down will add to the amount of time it will take to get them to stand. And since this method allows great air circulation you don't need to worry about letting the ear canal dry out or air out.
As you progress, the ears will be standing for a few hours when you first take the supports off. If by the next day they are still standing and look PERFECT, you can wait to put the supports back on. Watch the ears closely - once the tips start to soften and droop a bit* it is time to put them back up. Sometimes when a puppy is teething the ears will seem to get worse and not stand as well as they were a few weeks earlier. Don't worry, this is normal.
When the ears are starting to stand on their own it can be really tempting to leave the ears down to "see what they'll do". But don't do it - you will regret it later!
Eventually, the happy day will come when you take them down, they look great and stay that way!
*Sometimes, especially with a very long crop, the bases of the ears will be nice and solid and the dog is holding them up perfectly LONG before the very tips are strong. There isn't anything wrong with continuing to tape the entire ear until the tips are solid. But sometimes you can support just the tips - there are many methods for this including using Breathe-Rite strips, or just wrapping the tips with some tape. You can experiment and see what works for your dog.