I’m a rising senior, and I just took the June ACT. I got a 31c (34R, 33E, 29M, 29S) and a 23 for writing. My composite score was two points higher than the practice test I took because I did 6 points better on reading.
I think I could do better, and I’m considering taking it in September for a few reasons. My first concern is with the differences between English and Reading vs. Math and Science. My second concern is the writing score, which is in the 83rd percentile while my composite is in the 93rd. My third motivation to retake is that a 32 would put me in the highest scholarship bracket for a lot of scholarships.
What do you think? Is it worth it to study over the summer? Could I still apply early action?
If you are going into college with a math or science major, then I could see why the lower Math/Science scores would be concerning, but if you’re thinking about a more non math/science (Say, business, which puts emphasis on communication skills) major, then I think your 31 will be just fine, especially with your high English/Reading scores, so don’t worry about it. All in all, studying over summer and retaking it in September should give you just enough time to apply to pretty much all Early Decision (which usually happens in Nov.) but maybe not all early action, which can happen in the middle of October. It’s worth studying over summer if you think you really could raise your math/science scores (so focus on those subjects), but make sure to also focus on preparing your application and choosing colleges - that’s an important part of the summer before senior year too. Whatever your decision is, good luck! 
Agree with the above post ^^^.
Retaking will also depend upon which schools you are targeting and where your ACT falls in the schools percentiles: Bottom 25%, Mid 50% or Top 75%.