Installing existing iPhone 6 apps, etc. on new iPhone 6s. How can I do this?

[iMore Question]

Installing existing iphone 6 apps, etc. on new iphone 6s.

Pre-ordered iphone 6S as a T-Mobile customer. Phone will arrive by mail. If I back up my iphone 6 to iTunes, and then do a restore to the new iphone 6S (to install apps, etc), won't ios 8.4 override or replace ios9 that the new iphone ships with? Also, where is the best 'how to' to help me in this process, as I'm not super savvy at this stuff. Thanks for your considerations... All are welcome.