Hi ezhang167,
First off, a 750 is an outstanding Score, so you can comfortably apply to ANY Business Schools that interest you. Since many Business Schools have publicly stated that they don't use the IR score as part of the application process, you have nothing to worry about there. However, if you're interested in what a specific School/Program thinks about the IR score, then you should research that School directly (you might also consider contacting the School to ask). Either way, an IR6 is an above-average result for that section, so that particular piece of your GMAT Score really isn't much of a reason to retake the entire Exam.
Since you're interested in applying to some highly-competitive Schools, 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/GMAT assassins aren't born, they're made,
Rich