Trying to edit a master timesheet to spread across all other sheets in work book.

Hello all!

I have a time sheet that was made by an old employee (from what my boss told me, don't know exactly). I have not really found anything useful across it, but I think I am missing something. I have to change some items on it but I am trying to figure out if I can change them all at once or if i have to go page by page. The spread sheet I am attaching has been edited on the page labeled 20. It is a neat workbook, but I don't want to mess it up really. I want to learn from this so I could get better at excel.

So my questions are,
1 Can I edit all of the pages at once so I don't have to go page by page?
2 What other formulas are used besides the =Sum?
3 How does the time get put into the cell?

Thank you for the help.