Impossible. It’s actually a paradox.
If you knew then what you know now…whatever you know now would be built upon what you knew then.
Consequently, you would know more now than you did then. Meaning that you no longer knew then what you know now.
And if we assume that it’s a perpetual cycle, where you always knew then what you know now, then you would either know everything or have learned nothing.
You can only know something because you learned about it, typically through trial and error, and sweet, sweet failure.
Stop wishing you knew it then, be glad you know it now.