We learn from Genesis 1:1 that God “created the heavens and the earth.” The two terms in that verse, heavens and earth,
form a merism, where these two contrasting entities represent the totality of what God made. So we could understand that the new heaven and a new earth (Revelation 21:1) form a merism indicating a “new all.” Though this understanding is undoubtedly true, the flow of thought in the present paragraph compels us to focus on the relationship between heaven and earth. We are to understand that every aspect of that relationship is made new (= profoundly, qualitatively changed). Of course, if God could create heaven and earth in Genesis 1:1–31 with a word, he can certainly renew all things with ease.
5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.”