Here is the methodology for editing an entire field in ArcGIS (rather than editing each row individually). It can be done as follows:
1) Click Start EDiting on the editing tools
2) Click the attributes icon on the editor toolbar
3) Right click the layer which contains the attribute that you wish to mass edit at once
4) go to Select > Select All Features
5) Return to the attributes window and select the field you wish to edit (ensuring that you have selected the entire feature rather than just one entry)
6) insert the value you want
Now every shape in the shapefile should have the same attribute in the same field.
