Tuturu! What is the difference between episode 23 and the alternate version?

The first 15 minutes of the episode are identical to the original Episode 23. Rintaro Okabe travels back in time with Suzuha Amane to save Kurisu, but he accidentally stabs her himself, causing him to return to the present in a state of total emotional collapse. The divergence occurs in the final minutes:

Released in 2015 as a special promotional episode during a rebroadcast of the original anime, it serves as a prologue that explains why the "Steins Gate" world line—a future without World War III or the death of Kurisu Makise—did not happen immediately. The Point of Divergence: Why Episode 23β Matters

(often called Episode 23β) is the critical bridge that connects the original Steins;Gate series to its sequel, Steins;Gate 0 .