1. On the left-hand side, there are multiple options (Accounts, Places, Routes, Map Layers and Lasso). Click the lasso option.
2. Lasso selection should now extend a bit stating, “ Select Some Points!”
3. Above the comment “Select Some Points!,” displays the lasso icon “Select.” Tap this option.
4. Now, you are able to draw on the map (to your right) according to which locations/ accounts you want to include on your routes.
5. After drawing on the map, the accounts you have selected should be displayed on your left hand side in a list.
6. Below this list, there is the option to mass update in the bottom of the page. Click on it.
7. A pop-up should appear. You can change whatever attributes you like. For instance, click on "Select or create" below Priority and change the priority to "High" for all the accounts you've selected.
8. Click "Update Accounts" and you're done.
You can also watch this video: