Hi reviewtattantat,
If the Average GMAT Score of the School that you are interested in is a 550, then with a 620, you certainly appear to be in a good position to apply (as far as that one aspect of your application is concerned). You're ultimately asking an Admissions question though, so you would likely find it beneficial to speak with an Admissions Expert about your overall profile and plans. Those Experts should be able to answer your Admissions questions and help define the specific areas of your profile that could use some improvement.
There's a Forum full of those Experts here:
https://gmatclub.com/forum/ask-admissio ... tants-124/If you do choose to retest though, then we should take a closer look at how you were studying before. Once you've decided on whether to retest or not, you should post back here (or you can PM me directly) and we can talk through your study options going forward. If you took your Official GMAT at a Test Center, then you might also choose to purchase the Enhanced Score Report. While the ESR doesn’t provide a lot of information, there are usually a few data points that we can use to define what went wrong on Test Day (and what you should work on to score higher). If you purchase the ESR, then I'll be happy to analyze it for you.
GMAT assassins aren't born, they're made,
Rich