Is it possible employees to change information in department of water and power computer?

No.  That's why they have old phone numbers and in some cases old addresses for clients.  Once it's on the computer, that's it!

If the employees could not change anything the programs - such as marking that you paid a bill, update your contact information, adjust the settings on a piece of equipment, etc. - then they would not be able to do any work.  Of course they can update things on the computers.

Now, if you have a dispute with them, whether they can update information so that they can alter things to lie about something, over charge you, alter records, etc., is another question.  Usually if any changes are made to a system like that, the information is logged and it is easy to tell that it was altered after the facts.  

Plus only certain people are going to have the rights to make changes in something that could affect billing or make changes that could have legal consequences.

I'm sure some of the employees in the IT department could.