In "Back to the Future Part II," Biff Tannen uses the Grays Sports Almanac to alter history by giving it to his younger self in 1955. This act creates an alternate timeline where Biff becomes wealthy and powerful. However, the almanac itself doesn't change or update with each altered event. Instead, it remains a static record of sports statistics from the original timeline.
The mechanics of time travel in the "Back to the Future" series suggest that changes to the timeline don't retroactively alter existing objects. For example, when Marty and Doc return to 1985 and find it transformed into a dystopian version due to Biff's actions, the physical items they brought with them from the original timeline remain unchanged. Similarly, the sports almanac retains its original content, even though the events it describes may have unfolded differently in the new timeline.
This concept is further illustrated by the fact that photographs and newspapers in the series change to reflect alterations in the timeline, but objects that have traveled through time, like the almanac or the DeLorean, do not. Therefore, the almanac doesn't update dynamically with each change; it serves as a fixed artifact from the original timeline, providing Biff with the information he needed to amass his fortune in the altered 1985.
In summary, while Biff's use of the almanac creates an alternate timeline with significant changes, the almanac itself remains an unchanging record of sports events from the original timeline.