This is described about a quarter the way into TFA.
> REINDEX INDEX CONCURRENTLY index_name;
(A vanilla REINDEX will lock the table, preventing writes, while it runs. The CONCURRENT creates a new index, replicates any updates to the original while it does so, and then does an atomic switcheroo at the end.)
> REINDEX INDEX CONCURRENTLY index_name;
(A vanilla REINDEX will lock the table, preventing writes, while it runs. The CONCURRENT creates a new index, replicates any updates to the original while it does so, and then does an atomic switcheroo at the end.)