The monthly upload limit is similar to a cell phone data plan – meaning that once it's used, there is no way to undo that usage.
For example, if you're on the Part-Time plan, you can upload 10GB worth of files per month.
Once you've used that allotment, you cannot upload anything else for the rest of the month, even if you delete some files from your account.
The upload limit resets every month based on the day your paid plan started – if you're unsure, you can visit the billing page to see your renewal date. The day of the month that your plan renews will be the day your upload limit resets each month.
If you need more bandwidth than your account provides, please consider switching to a higher plan (read: how to change plans).