The birthday cake had so many candles Jessica could barely blow them out.
In this context, "them" refers to the candles. However, if the goal is to replace the underlined words while maintaining coherence, we could replace "Jessica" with "she" (the correct pronoun referring to her).
So the revised sentence would read:
"The birthday cake had so many candles she could barely blow them out."
If you want to change "it," you would need to replace the appropriate context. Please clarify if you need to replace other specific sections instead!