Exactly what the title says.
Personally, I don't really think that ten things being changed is enough to make this movie is awesome as the original, even though it's pretty decent by Disney sequel standards, but I'll try my best!
1. Have the movie focus more on Simba's struggle to live up to his fathers image, though I'd still give Kiara and Kovu plenty of screen time
2. Better animation and have the characters designs be like the original
3. I wouldn't change Kovu's general character much, but Kiara definitely needs to be rewritten. I would either 1. Make her smarter and less eager to trust Kovu, especially at first, or 2. Keep her a naive airhead, but make it clear that Simba sheltering her is the reason why she is this way and have her mature as the movie goes on. I'd also at least mention her "I don't wanna be queen!" thing as a cub
4. Give Nala more screen time, and, y'know, a personality like the one she had in the original. Maybe a scene where she and Sarabi talk to Kiara?
5. Completely axe Upendi, and replace with a montage of Kiara and Kovu interacting, or something like that. That way we not only have more time for Kovu to convert, making him more realistic, but also no more Upendi! Killing two birds with one stone!
6. Also completely axe Nuka and have Vitani fill his role instead
7. Way, way, WAY less Timon and Pumbaa. I'm sorry, but I really hate them
8. Most of the deleted scenes (especially Zira's suicide and Nuka's final words) put back in
9. Umm, have Kovu actually help Simba save Kiara at the end. You are supposed to be a trained super lion, for roaring out loud
10. Give the outlanders an actual backstory.