Edit default alert in subscribed calender
On a Mac running a current OS, this is how you would go about adding multiple alerts for one event.




Double-click your event
Click the date/time of the event
Click the 'Alert' field and select when you would like your first alert
to happen (notice that there is a 'custom' field)
Now that you've selected a time period for your first alert, if you
hover over the 'Alert' field you should see a small '+' symbol to the right; if you click it, it should trigger a second alert option.


On an iPhone:


Locate your event and tap it to open it
Click 'Edit' in the top right corner of the screen (Note: If you don't click edit and instead only try to adjust the alert section from the main window, it will not give you the option to set up a second alert.)
Scroll down to the Alert section; choose the time frame for your
first alert
Now that you've set the first alert, you should see an option for
'Second Alert' below the original 'Alert' option (Note: The 'Second Alert' option doesn't look like it exists until you set up the first alert)
Choose a time frame for the second alert and then click 'Done' in the
top right hand corner of the screen