100mg/1ml is 10 percent nic...6mg/ml is .6 percent nic. That is a HUGE difference...I wouldn't be working with anything over 24mg nic for safety till you make a few batches and really get the hang of basic mixing...just my two cents...
There is no realistic way to undo what's been done if I understand the OP correctly. Anyone who is starting out with DIY should start with a premix base of their desired level....so if you want 12mg liquid than buy 12mg base. DIY can seem simple at first glace but it is really easy to make a mistake and overnic yourself effectively poisoning yourself of something worse. And 100mg nic is nasty stuff...if you don't take the proper precautions like wearing gloves and working in a well ventilated area you can poison yourself just from the fumes or skin contact if you don't have a really high tolerance to begin with. I gave myself a few headaches when I was starting out thinking I would be fine and didn't need to worry about gloves and stuff. Stupid lesson learned the hard way.
Unforchinately...if I understand the OP right...you mixed up a liter of 6 mg liquid and there isn't really an easy way of saving it....you'd have to add more nic and then add more flavor and more pg/vg and you're working with such a huge amount of everything that you'd more than likely end up wasting more than you have trying to balance everything out trying to get the flavor right and it's safer to just scrub it and start over.. Always start off making small batches...like 10 or 15 ml. I've still got bottles from a year ago I had to throw away that I was sure I would vape threw that the nic level is too high and I can't vape the stuff now without getting a headache.
DIY is all about baby steps...small mixes....flavors are going to change greatly over the first two weeks if the bottle lasts that long from steeping alone. Always make sure you have a nic level test kit and know how to use it. You can purchase one from these places.
http://www.eliquidtest.com/
http://nudenicotine.com/
It's not as common as it used to be but there was a time that people would order one nic level base and they would get something different from what they ordered. I am not saying you can not trust vendors, I am saying better to double check everything and be safe rather than sorry. And it's good to check your mixes as well to make sure your math is on, even if you use a calculator....Safety my friend. Better to ask questions, no matter how far fetched they may seem, and double check things and be safe than end up poisoning yourself.