You can edit the Base Symbology for multiple objects within a Base(vector) layer by using the steps below:

1. Select your Base(vector) layer in the Contents window.
2. In the Properties window, select the General tab.
3. Click the Edit Symbology button. The Symbology dialog opens.
4. Assign Unique Values using your preferred Attribute field. More detail on how to do this is available in this article: Color code objects in a base layer by attribute value.
5. Hold CTRL and click to select multiple entries that you wish to modify as a group. Selected lines will be highlighted in blue (as seen below). Alternatively, select all the entries by holding SHIFT and clicking the first and last line items.
8. In the right hand pane, you can now edit the properties such as Foreground color and Pattern of the selected items.
8. If desired click the Save button to save this symbology as a file that can be applied to future projects with matching attributes. Click Load to import a saved symbology file. 
9. Click OK when finished.

 Updated March 2020