The secondary offer rate is the gross interest rate you choose to buy loan parts at from other investors.
You can change the rates you want to buy loan parts at using Autobid, by simply turning Autobid off and clicking on advanced settings. You can adjust your offer rates for each risk band here. You should check your account over time to ensure that your offers are still competitive vs. other investors.
Once a bid has been made on Funding Circle, however, it cannot be altered whether made on an individual loan request or through Autobid.