we need reinstall apt and upgrade it.
2. Download libapt-pkg5.0_1.3~pre3_*.deb ( where * is your required architecture )
3. Download apt_1.3~pre3_*.deb ( where * is your required architecture )
4. Open a terminal, navigate to the directory of the downloaded files above
5. Uninstall apt by apt-get remove apt, and record the packages to be removed
6. Install libapt-pkg5.0_1.3~pre3 by dpkg -i libapt-pkg5.0_1.3~pre3_*.deb
7. Install apt_1.3~pre3_*bby dpkg -i apt_1.3~pre3_*.deb
8. Run apt-get update again, it should work by now.
9. Install the packages removed in step 5.