The next step is to go to your submission, read the comments on what has to be improved and take a note of the granted extra upload time. For example, grace period until 21/07/2015 UTC 2359 means that you have the option to make the required changes and re-upload your entry by this time.
Please note that grace period is only available for some contests. Most contest do not have this feature as clients need the submissions urgently. It is also not compulsory to improve on your submission as requested but it is highly recommended so that your effort in creating your work will not be wasted.