This is a bios setting. What motherboard do you have? They should have information on their website how to change it. I did this recently to my 2200G and Asrock B450 Pro4, and BOINC definitely used the additional memory for GPU tasks. Keep in mind a few things, though:
1. I was only able to increase allocation to 3 Gb. Beyond that I need to disable compatibility support module (CSM) in bios. When I would try that, I would just boot into bios every time, I couldn’ boot into Windows.
2. I also just bought a discrete GPU, and I suspect that by changing a setting in bios to allow more DRAM to be allocated, I couldn’t post. I had to remove the GpU and clear the CMOS jumper to get it to work again. I haven’t had a chance to try installing the GPU again so I can’t prove that is the problem, but just be warned if you do have a discrete GPU as well.