Microsoft never provided a rollback path for Xbox 360 dashboard updates. Once you accept a system update, the previous dashboard is gone—permanently. Here's the technical why and what (little) you can do.
The "New Xbox Experience" (NXE, 2008), "Metro" (2011), and final "Modern" (2015) dashboards each killed the prior. If you miss blades, channels, or the original guide—you're not alone. But the hardware enforces forward-only.
No official downgrade. No safe unofficial one. The dashboard you have is the dashboard you keep.