There are 2 ways to approach this.

The first is manual selection, the other as schedules.

Manual selection

Hoover with your cursor over the pipe or duct. Now press tab multiple times so that you select all the pipes. In the ribbon you can now change the diameter to the one you want.

Fittings will change with this method, but it will change all the selected pipes/ducts into the chosen diameter. This is not a very suitable method for when you have a complex system with loads of different diameters.

Schedules

Make a schedule for the category that you need (for pipes, make a pipe schedule).

As fields, add the Family, Type, System, System Name & Diameter.

As sorting, make sure to sort on either system type or system name (depends on your method of systems in Revit). Sort on family, type and diameter. Uncheck the Itemize every instance.

You'll get a nice list of pipes sorted on system, type and diameter. If you change one diameter to another, it will automatically change all the included diameters to the chosen one.

Keep in mind that fittings won't change diameter and you will have to look through all the fittings. This is a method that works if you have a lot of different diameters in a single system.

For the fittings you could create a seperate schedule and either use the highlight in model or remove the ones from the project and use the warning dialogue in Revit to find all open ends to pipes/ducts and solve them that way.