First try to install the new driver:
apt-get install nvidia-legacy-340xx-driver libgl1-nvidia-legacy-340xx-glx-i386 libgles1-nvidia-legacy-340xx libgles2-nvidia-legacy-340xx
Reading package lists... Done
Building dependency tree
Reading state information... Done
libgles1-nvidia-legacy-340xx is already the newest version.
libgles2-nvidia-legacy-340xx is already the newest version.
nvidia-legacy-340xx-driver is already the newest version.
libgl1-nvidia-legacy-340xx-glx-i386:i386 is already the newest version.
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
libegl1-nvidia-legacy-340xx : Depends: libnvidia-legacy-340xx-eglcore (= 340.76-3) but it is not going to be installed
libgl1-nvidia-legacy-340xx-glx-i386:i386 : Depends: libgl1-nvidia-legacy-340xx-glx:i386 but it is not going to be installed
libgles1-nvidia-legacy-340xx : Depends: libnvidia-legacy-340xx-eglcore (= 340.76-3) but it is not going to be installed
libgles2-nvidia-legacy-340xx : Depends: libnvidia-legacy-340xx-eglcore (= 340.76-3) but it is not going to be installed
nvidia-legacy-340xx-driver : Depends: libgl1-nvidia-legacy-340xx-glx (= 340.76-3) but it is not going to be installed
Depends: nvidia-legacy-340xx-driver-bin (= 340.76-3) but it is not going to be installed
xserver-xorg-video-nvidia-legacy-340xx : Depends: libgl1-nvidia-legacy-340xx-glx (= 340.76-3) but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).
Using 'apt-get -f install':
The following extra packages will be installed:
libgl1-nvidia-legacy-340xx-glx libgl1-nvidia-legacy-340xx-glx:i386 libnvidia-legacy-340xx-eglcore
nvidia-legacy-340xx-driver-bin
The following NEW packages will be installed:
libgl1-nvidia-legacy-340xx-glx libgl1-nvidia-legacy-340xx-glx:i386 libnvidia-legacy-340xx-eglcore
nvidia-legacy-340xx-driver-bin
0 upgraded, 4 newly installed, 0 to remove and 10 not upgraded.
10 not fully installed or removed.
Need to get 0 B/22.2 MB of archives.
After this operation, 130 MB of additional disk space will be used.
Do you want to continue? [Y/n]
Preconfiguring packages ...
(Reading database ... 329749 files and directories currently installed.)
Preparing to unpack .../libnvidia-legacy-340xx-eglcore_340.76-3_amd64.deb ...
Unpacking libnvidia-legacy-340xx-eglcore:amd64 (340.76-3) ...
dpkg: error processing archive /var/cache/apt/archives/libnvidia-legacy-340xx-eglcore_340.76-3_amd64.deb (--unpack):
trying to overwrite '/usr/lib/x86_64-linux-gnu/libnvidia-glsi.so.340.76', which is also in package libnvidia-eglcore:amd64 340.76-1.siduction.1
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Preparing to unpack .../libgl1-nvidia-legacy-340xx-glx_340.76-3_amd64.deb ...
Unpacking libgl1-nvidia-legacy-340xx-glx:amd64 (340.76-3) ...
dpkg: error processing archive /var/cache/apt/archives/libgl1-nvidia-legacy-340xx-glx_340.76-3_amd64.deb (--unpack):
trying to overwrite '/usr/lib/x86_64-linux-gnu/tls/libnvidia-tls.so.340.76', which is also in package libgl1-nvidia-glx:amd64 340.76-1.siduction.1
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Preparing to unpack .../nvidia-legacy-340xx-driver-bin_340.76-3_amd64.deb ...
Unpacking nvidia-legacy-340xx-driver-bin (340.76-3) ...
dpkg: error processing archive /var/cache/apt/archives/nvidia-legacy-340xx-driver-bin_340.76-3_amd64.deb (--unpack):
trying to overwrite '/usr/share/nvidia/nvidia-application-profiles-340.76-key-documentation', which is also in package nvidia-driver-bin 340.76-1.siduction.1
Preparing to unpack .../libgl1-nvidia-legacy-340xx-glx_340.76-3_i386.deb ...
Unpacking libgl1-nvidia-legacy-340xx-glx:i386 (340.76-3) ...
dpkg: error processing archive /var/cache/apt/archives/libgl1-nvidia-legacy-340xx-glx_340.76-3_i386.deb (--unpack):
trying to overwrite '/usr/lib/i386-linux-gnu/tls/libnvidia-tls.so.340.76', which is also in package libgl1-nvidia-glx:i386 340.76-1.siduction.1
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Errors were encountered while processing:
/var/cache/apt/archives/libnvidia-legacy-340xx-eglcore_340.76-3_amd64.deb
/var/cache/apt/archives/libgl1-nvidia-legacy-340xx-glx_340.76-3_amd64.deb
/var/cache/apt/archives/nvidia-legacy-340xx-driver-bin_340.76-3_amd64.deb
/var/cache/apt/archives/libgl1-nvidia-legacy-340xx-glx_340.76-3_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)