How to Disconnect from a database and go back to the default database in PostgreSQL?

It's easy, just look the example.

--my databases

postgres=# \l
                               List of databases
   Name    |  Owner   | Encoding | Collate | Ctype |     Access privileges     
-----------+----------+----------+---------+-------+---------------------------
 francs    | postgres | UTF8     | C       | C     | =Tc/postgres             +
           |          |          |         |       | postgres=CTc/postgres    +
           |          |          |         |       | francs=C*T*c*/postgres   +
           |          |          |         |       | select_only=c/francs
 postgres  | postgres | UTF8     | C       | C     | 
 source_db | postgres | UTF8     | C       | C     | =Tc/postgres             +
           |          |          |         |       | postgres=CTc/postgres    +
           |          |          |         |       | source_db=C*T*c*/postgres
 template0 | postgres | UTF8     | C       | C     | =c/postgres              +
           |          |          |         |       | postgres=CTc/postgres
 template1 | postgres | UTF8     | C       | C     | =c/postgres              +
           |          |          |         |       | postgres=CTc/postgres
(5 rows)

--switch to db francs as role francs

postgres=# \c francs francs
You are now connected to database "francs" as user "francs".

--swith to db postgres as role postgres

francs=> \c postgres postgres

You are now connected to database "postgres" as user "postgres".
postgres=# 

--disconnect from db

postgres=# \q

There is not a 'disconnect' in psql. Instead of disconnecting from your newdb database you connect with the default postgres database.

Create the new database and connect to it:

postgres=# create database newdb;
CREATE DATABASE    
postgres=# \c newdb
You are now connected to database "newdb" as user "postgres".
newdb=#

List the number of connections on newdb:

newdb=# select datname,numbackends from  pg_stat_database where datname='newdb';
 datname | numbackends
---------+-------------
 newdb   |           1

Now, instead of disconnecting, just connect with the default postgres database.

newdb=# \c postgres
You are now connected to database "postgres" as user "postgres".
postgres=#

Now there are no connections on newdb:

postgres=# select datname,numbackends from  pg_stat_database where datname='newdb';
 datname | numbackends
---------+-------------
 newdb   |           0