Right time to make changes??

If my Bill is due on the July 25th of each month, but my bill was generated on July 4th, and the bill cycle is from July 8th to Aug 7th ... when I can do the changes to my plan to avoid double charges in my billing cycle and to loose any paid access???

Very bad customer service Xfinity!!! Cannot  talk to anybody to explain anything, very deceiving and confusing bill, information on prices, etc.