I have already filed for opt stem extension from the e-verified company I am working at currently. Having been interviewed a couple of times by another company, it's highly likely that I will be accepted for a better job. However, on my stem extension OPT I20, it lists my current company as my employer.
1. How can i let USCIS know if I join another company while my extension is still in progress and hasn't be approved yet?? 
2. Would letting the USCIS know about this not delay my extension by some more time??

I'd appreciate any suggestions and previous experiences. Thanks guys!!