How do I get up to the next level in Duolingo? I'm on level 15 in French with all icons orange. How do I get to level 16?
Just keep strengthening and repeating the lessons, you'll still be getting xp and level up
XP shouldn't be your main focus, just wait for your skills to go down and strengthen them when necessary, one day your level will increase. Try using your French in other contexts than Duolingo, you can for example use Memrise for vocabulary, but also listen to music, watch movies, read novels, speak with natives, etc...