This looks to me to be the normal fade of the plant... and at 9 weeks old, you're coming to the end of an autoflower's life span... If you want to give it a small dose of bloom nutes, it wouldn't hurt it at all - but you're not going to see any improvement in the leaves that are yellowing. Whatever you've been doing to keep that soil going for this long has been really good and I congratulate you! I think you'll have a really nice harvest...
Good luck!