You click on your mouse, then click the settings wheel at the top right. This should then give you the ability to turn the onboard memory "on" and then you can sync any profiles you have created to any of the 5 slots.
If you only have "default", then you need to run off on-board memory, go back to the main page where you see a big picture of your mouse and click the profile name at the top. You should then be in a window where you can create new profiles. You simply give them a name. Then go back to the main page and click your mouse. You can then use the drop down list top left to select your profiles and customise them. Once you are done, go back to the first paragraph above and sync them to your mouse.
Logitech made this interface insanely clumsy. I don't think I've ever witness such a non-user-friendly interface, but that is how you do it.
I am not sure how to remap the DPI button. It isn't possible for me at least and I have the G502 mouse.