(update 02-18-2019) Came across this blog post by Jonathan Li, which has more comprehensive steps.
(Original 03-11-2017) Recently I picked up my old iOS app as the framework it replied on Parse was going sunset (the date was Jan 28, 2017). I picked up Google firebase after reading a few posts including the one at Raywenderlich.com. In the process I also upgraded CocoaPods, and replacing the ads from iAd to admob. About 3 months passed by since the initial launch of admob, it seems the performance of admob is much better than the iAd. Note I only used the banner ads for now: this is similar to the iAd banner ads it replaces with. Per Matthijs Hollemans (the famous iOS dev, also raywenderlich.com tutorial writer), interstitial ads is more powerful when possible, but I have not successfully made it work yet.
Going forward, I am thinking learn about the Swift language, as David Smith and Marco Arment mentioned in recent Under the Radar podcast (State of Swift), this is increasingly urgent now, for iOS native app dev.
Back to ads, for me I have seen the steady declining of my blog website viewership, and the income from google ads. This admob thing gave me some hope. This also confirms the trend of people increasingly switching from desktop to mobile, esp. smart phone.