There is a doctrine called the Immutability of God. It means that God does not change. God is the same yesterday, today, and tomorrow. That means the God revealed in the Old and New Testaments is the same God.
What has changed is the revelation of who God is and the covenant between God and man. With the incarnation, life, death, burial, resurrection, and ascension of the Lord Jesus Christ, God was fully revealed.
God also raised up the apostles to record all the information about who He is, what He has done, and who we are in Jesus. The Bible is a book of progressive revelation about the God of all creation who is eternal, without change.
Read more: How Can We Be Certain the Bible was Inspired by God? Learn the Bible: Books of the Bible-Fast, Fun, Short Video Summaries