You got a letter from your ED or EA school that indicates their decision to defer your decision until a later round. What should you do?
Take a deep break and think before you act.
If you applied ED, you will likely still want to be considered. Note that a defer decision does enable you to apply ED II to another school, if desired, as you are no longer bound by the ED agreement since you were not admitted during that round. However, if you get accepted to another school ED II, you will forgo your chance of enrolling in the initial institution if you are ultimately accepted through the Regular Decision pool.
If you applied EA somewhere, there is really no downside in being considered in the Regular Decision pool - they want to take a look at your application within a larger pool of applicants. The only reason why you would decline this opportunity is if you are no longer interested because you were admitted to another school or schools that you like better than this one.
If you opt to have your application reviewed in the next round, take the time to review the fine print - what does the communication say? You will often be asked to formally confirm your interest in being considered during the Regular Decision round. You may need to do this through the school's portal, through an email, or through a form.
Review what other information the school may be asking of you - do they want to see your fall semester senior year grade? Do they ask about your test scores? Do they allow you to send another letter of recommendation or updated extra-curricular involvement? Will they permit you to send a letter of continued interest?
If you are unsure what their policy is regarding additional materials and it is not clear on their website, reach out to your admission counselor to confirm. The last thing you want to do if you are in that "maybe" category is to do something that will annoy them or give them a reason to push you to the "deny" pool.
A defer decision at this stage means that you are competitive enough to be considered for admission, but they want to see how you stack up with the rest of the applicants to the college/university. Typically, showing your continued interest and providing new information can absolutely help you - just do it the right way. There is no use in rushing to send information NOW. They just reviewed your application. Send an update when you have compelling new information to share. You know they aren't going to review the application again until the Regular Decision applications have been submitted. The end of January/beginning of February is typically a good time to provide updated grades, new information about your extra-curricular involvement, and perhaps share your plans for the summer. If they allow for another letter of recommendation, you can send this at that time as well.
Good luck!