Removing Pip's Cache?

 I need to install psycopg2 v2.4.1 specifically. I accidentally did:

pip install psycopg2
Instead of:
pip install psycopg2==2.4.1
That instals 2.4.4 instead of the earlier version.

Now even after I uninstall psycopg2 and attempt to reinstall with the correct version, it appears that pip is re-using the cache it downloaded the first time. How can I force pip to clear out its download cache and use the specific version I'm including in the command? Please guide how to clear the pip cache?