What Rolling Stone reports him saying:
“But I beat him in the first debate. At the second debate, we had a never-Trumper as a host, but that’s OK because I beat [Biden] in the second debate also.”
What he actually said, according to the official transcript:
"But I beat him in the first debate. At the second debate we have a never Trumper as a host, but that's OK because I'd beat him in the second debate also."
You can hear for yourself:
https://www.youtube.com/watch?v=0Po84VhQjUA
01:14-01:24
"I", "I'd". You decide, but sounded pretty clear to me.