Download OOTP25 from here:
https://www.ootpdevelopments.com/ootp-prior-version/
Then install it. From the main page, import the 24 game into 25 (doesn't require a license, you just stay on that main page).
Then go to OOTP26 and import that OOTP25 game you just made.
That prior-versions-of-the-game site is made for folks needing to walk up their games multiple versions.