If deleting /var/lib/apt/lists/* does not work...
(esp. if you're behind a proxy), fix "Hash Sum Mismatch" like this:
Create file
/etc/apt/apt.conf.d/99fixbadproxy
with this content
Acquire::http::Pipeline-Depth 0;
Acquire::http::No-Cache true;
Acquire::BrokenProxy true;
See also here