If that text is inside the normal post content, you usually can’t change just one small paragraph directly in Update a Post.
The simple way is:
Get the post first
Take the current content
Replace only the text you want (example: old price → new price)
Send the full updated content back into Update a Post
So the flow is:
Get Post → replace text → Update Post
WordPress updates posts through its REST API, and posts include fields like content that get updated when you send the new version back.
In Make community examples, people use the same idea: get the post content first, change the needed part, then update the post again.