How do I increase dedicated video card

The memory on a video card or integrated video cannot be changed.  The 8 GB of ram is computer memory, not video memory.

Always include your exact model's name and number. Lots of XPS 15 models available. 15 is the size of the screen.

To my knowledge there's no way to control this.  The figures you're looking at are for the Intel GPU, which is built into the CPU but doesn't have any physical memory allocated.  It relies entirely on system memory.  The "dedicated" number you're looking at is the amount of system memory that is always reserved for the GPU, no matter what.  The shared memory is the amount of system memory that COULD be used by the GPU if the system doesn't need it for something else.  To my knowledge, you don't get to control how the system allocates memory between system and GPU usage.  But in general, if your system needs memory for something, it will perform better that memory is actually allocated to the system instead of the GPU anyway.

All that said, if your XPS 15 configuration also has an NVIDIA GPU, that GPU would have its own dedicated memory, which is physically separate from system memory and always dedicated to the GPU.

How can I check if the GPU has its dedicated please? 

Thanks a lot. However the RAM is 16GB. It's the graphics card that is 8GB

Open the device manager. If you have discrete video - which from your description is unlikely - you'll see two entries:  one for the Intel GPU and one for the nVidia or AMD GPU.

@Unphantomable Just because you have 16GB of system RAM doesn’t mean your Intel GPU would ever use that much. If it shows as 8GB shared, then that means that’s the most it would ever use, which is completely reasonable for integrated GPUs given that 8GB is still a completely reasonable amount to have in current desktop GPUs.

As to your question about how you can tell if you have a discrete GPU in your system, I consider it a bit strange that you’re worried about VRAM and don’t even know whether your system has a discrete GPU at all, since that would make a much, MUCH larger impact on graphics performance. But as mentioned, check Device Manager. If you have an NVIDIA GPU, then open NVIDIA Control Panel AMD in there you should be able to find information about how much memory that GPU has.