The only way I know to do it is to edit the input file directly. You
can open the .net file in Excel and do this. You can even sort within
major groups, but don't mix up the groups. You can then find the
pipes you're looking for (easiest if you name them so that they will
appear together in a sorted list) and change the pipe diameters by
pasting a block of cells or dragging or whatever.
Clarification: you can't edit the .net file directly. In EPANET, use
the File>Export>Network command to export a .INP file. This can be
opened in Excel and makes it much easier to mass edit parameters.
Then this file can be imported using the File>Import>Network command.