How would you change splendid into a noun?

You can't directly change "splendid" into a noun. It's an adjective describing something as magnificent or impressive.

Here's how you can make it into a noun:

* Use it as a noun phrase: "The splendidness of the sunset was breathtaking."

* Use a related noun: "The splendor of the palace was awe-inspiring."

* Use a synonym as a noun: "The magnificence of the performance left the audience speechless."

The best choice will depend on the specific context you're using it in.