Hi frostorm,
First off, a 690/Q50 is a fantastic score (it's just a bit below the 90th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. Depending on the Schools that you plan to apply to, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
https://gmatclub.com/forum/ask-admission ... tants-124/1) When are you planning to apply to Business School?
2) What Schools are you planning to apply to?
If you insist on retesting, the logical first start would be to take a NEW, FULL-LENGTH CAT (under realistic conditions) so that we can get a sense of your current skills, strengths and weaknesses. Once you have that score, you should post back here and we can discuss how best to proceed.
GMAT assassins aren't born, they're made,
Rich