The following code update the page title.
page = Page.find page_id # API access
print page.title # -> ABC\nDEF
tp = page.properties["Title"]
tp[1].text = "GHI"
page.save # API access
print page.title # -> ABC\nGHI
page = Page.new id: page_id, assign: [TitleProperty, "Title"]
tp = page.properties["Title"]
tp << TextObject.new("JKL")
page.save # API access
print page.title # -> JKL