I'm not sure that you can change the default for the new slide button, but
here's a potential workaround. Make a macro...
Go to tools>>>macro>>>record new macro (make sure that you have saved your
presentation before creating a macro).
Title the macro (a name of your choice that you'll remember), then hit "OK"
Now every keystroke you make will be recorded.
While recording, do this...
CTRL+M (to make a new slide)
Choose the layout you want (title, text, content)
Now, click the small square button in the macro recording window to stop
recording.
To place your macro on a toolbar so you can use it, do the following...
With your cursor over a toolbar, right click and choose "Customize"
In the new window that comes up, on the left side, scroll down until you see
"Macros"
Now your newly recorded macro should be listed in the right side. To move
this to a toolbar, click and drag the macro title from the window to your
toolbar.
Now, whenever you want that particular slide layout, just click this button.
Hope this helps!!!
