I just started the course, but figured the deadline is approaching. I already submitted solutions to some of the psets. I don't think I'm gonna be able to finish before the deadline. what should I do?
CS50 is taught in the fall of each year on campus. the same version, taught in the fall, becomes on edX in January in the year to follow.
as each version is taught on campus (even before they're on edX), the materials are up on https://cs50.harvard.edu/. you can totally access them. though you can't submit solutions to the psets.
you can have access to the materials of any version of the course through http://cs50.tv/. see the screenshot below:
regarding the psets, typically some of them get minor modifications from version to another. some of them are completely removed. others are added. in case a pset is the same as an old one you already solved, it is reasonable to resubmit your solution to that psets as long as it satisfies all the requirements. you don't have to solve it again, although you may wanna do that just as a refresher.