Frankly it depends on how it's slipping, which fingers are slipping and in what way and what hold over it (if any) it leaves you with. As you probably know a week or even at times a month isn't usually enough to find what you like or be good with it there, so just experiment on where you like your fingers. If you want to change your grip to one that may play better for you now somethings you can do are move where you hold the pen up or down, move your hand in relation to the tablet, and change how your fingers are wrapped around the pen. If you don't what to change any of that just follow the golden rule and play more you can make most anything work with time. And remember it's the player that makes the grip don't stress out too much over it.
Hope this helps and if you have any questions or complaints with what I've said I would be happy to address them however possible till then have a nice day or night