What is the difference in our Teshuva now in the month of Elul leading up to Rosh Hashana versus that of Aseret Yemei Teshuva before Yom Kippur. What is the difference in the significance of Rosh Hashana and Yom Kippur as Yamim Noraim?

Teshuva in the month of Elul is preparation for the Teshuva that we will do in the 10 days of Teshuva. It is getting use to the idea of doing Teshuva - a mental warm up. The more one prepares, the better the finished product is. Rosh Hashana is no about repenting on past sins. It is about making Hashem the King and realizing that Hashem is everything in our lives. That is why there is no mention of repentance in the Davening. Some people even omit Avinu Malkainu Chatanu Lefanecha. Yom kippur is about repentance on past sins, and accepting not to do them anymore. Rav Avigdor Neventzal has a very good essay on this topic in his book on Rosh Hashana.