postgres=# alter database template1 rename to template2;
ALTER DATABASE
postgres=# \l
                                 List of databases
   Name    |  Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+----------+----------+------------+------------+-----------------------
 postgres  | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 
 template0 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres
 template2 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres
(3 rows)

postgres=# create database template1 template template0 ;
CREATE DATABASE

postgres=# alter database template1 is_template=True;
ALTER DATABASE
postgres=# \l
                                 List of databases
   Name    |  Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+----------+----------+------------+------------+-----------------------
 postgres  | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 
 template0 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres
 template1 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 
 template2 | postgres | UTF8     | en_US.utf8 | en_US.utf8 | =c/postgres          +
           |          |          |            |            | postgres=CTc/postgres

postgres=# select * from pg_database ;
  datname  | datdba | encoding | datcollate |  datctype  | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfrozenxid | datminmxid | dattablespace |               datacl                
-----------+--------+----------+------------+------------+---------------+--------------+--------------+---------------+--------------+------------+---------------+-------------------------------------
 postgres  |     10 |        6 | en_US.utf8 | en_US.utf8 | f             | t            |           -1 |         13066 |          562 |          1 |          1663 | 
 template2 |     10 |        6 | en_US.utf8 | en_US.utf8 | t             | t            |           -1 |         13066 |          562 |          1 |          1663 | {=c/postgres,postgres=CTc/postgres}
 template0 |     10 |        6 | en_US.utf8 | en_US.utf8 | t             | f            |           -1 |         13066 |          562 |          1 |          1663 | {=c/postgres,postgres=CTc/postgres}
 template1 |     10 |        6 | en_US.utf8 | en_US.utf8 | t             | t            |           -1 |         13066 |          562 |          1 |          1663 | 
(4 rows)