The first option is to edit the pipe individually. To do this select the pipe, right click and select Pipe Properties and go to the Part Properties Tab. Change the size listed to the proper size for the pipe.
The second option is to edit multiple pipes at the same time. To do this select the Swap Parts command on the FDOT Toolbar then select all the pipes you want to include.
Click Enter and the Swap Parts dialog will appear. Verify the Part List is set to DREX, Pipe Part Family is Pipes for Existing Utility Lines and change the size. Click OK and all the selected pipes will change.