Make this your /etc/apt/sources.list.d/debian.list:
# debian loadbalancer
deb http://httpredir.debian.org/debian/ unstable main contrib non-free
# deb-src http://httpredir.debian.org/debian/ unstable main contrib non-free
# deb http://deb.debian.org/debian/ experimental main contrib non-free
# deb-src http://deb.debian.org/debian/ experimental main contrib non-free
# deb http://deb.debian.org/debian/ testing main contrib non-free
# deb-src http://deb.debian.org/debian/ testing main contrib non-free
When you want to install the nvidia driver from experimental repo, open /etc/apt/sources.list.d/debian.list for editing. Remove the "#" from the beginning of the line
# deb http://deb.debian.org/debian/ experimental main contrib non-free
and save the file.
Next, in terminal
apt update
apt-install -t experimental nvidia-driver
(let the installation complete)
later, you can open /etc/apt/sources.list.d and put the "#" back in front of the experimental repo line.