Locate a verified "sprites changed" APK from a reputable archive site.
As Rovio released more games, they sought a cohesive art style across the entire Angry Birds ecosystem.
Which you are trying to play on (Android, PC, or iOS)?
Swapping the movie characters (Blu and Jewel) with modern designs of Red and Stella.
As smartphones became more powerful, early optimizations that made graphics look "pixelated" on high-res screens were replaced by higher-fidelity assets, but sometimes the "charm" of the old, rougher sprites was lost.