Depends on your demographics and rest of you profile, but I would go ahead and cancel your score since you already have your test from last year, which is higher.

Realistically, I don't think you're going to make a significant improvement with 2-weeks of study. You might need more like 1-2 months depending on your target. If M7 is your goal, then you're looking at a GMAT average score around 720-740 to be competitive.

Because some schools are looking for Q45+ Quant scores, I don't think cancelling the score could be a good idea. The improvement in Quant is worth having on your record. As for when and whether to retake, that depends on a few things. You'd need to ask an admissions expert how competitive you'll be at your target schools with your current score (and that expert will need more information about your profile). If it turns out you decide to retake, you'd want to use the information you have from GMATPrep tests to decide when to take the test again. If you were consistently scoring 700+ on official diagnostic tests, there's every reason to think you'll do that on the real thing if you perform normally, so the 680 was probably just slightly bad luck. You could take the test tomorrow and be quite confident of a 700+ result. But if your diagnostic scores were more in the 660-690 range, you'd probably want to make some genuine improvement before retaking. It shouldn't take a long time, but it can be hard to make that kind of progress in only a week or so, unless you have a clear idea of what you need to work on. Good luck!
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