DECIGO is a future Japanese gravitational wave detector in space, designed to enhance gravitational wave astronomy. Operating within a frequency range of 0.1 Hz to 10 Hz, DECIGO plays a vital role in detecting gravitational wave signals from various sources, including primordial gravitational waves (PGWs). The pre-conceptual design of DECIGO features a cluster of three spacecraft, each equipped with lasers and floating mirrors, enabling precise measurements of gravitational waves. However, recent advancements in our understanding of the upper limit of PGWs have necessitated an update to DECIGO's design. The updated design aims to significantly improve DECIGO's sensitivity. Additionally, in preparation for DECIGO, a smaller version called B-DECIGO is planned for launch to test various technologies and achieve important scientific objectives. The ongoing development and updates in DECIGO's design demonstrate its potential to revolutionize gravitational wave astronomy and contribute to significant scientific discoveries.