“Looking forward to seeing it” is the correct one. “To” here is a preposition connected to the adverb forward, which is modifying the word looking. To is not connected to the word see as if it’s an infinitive. We must use nouns or gerund forms after the expression looking forward to. Examples: I’m looking forward to your graduation. (Noun) I’m looking forward to seeing you tonight. (Gerund)

“Looking forward to seeing it”is correct because “to”here is a preposition. It should be added a noun or a gerund.