Switching mySQL replication MASTER to SLAVE and SLAVE to MASTER

I use two mySQL 5.5 databases synced the one on the other but I'd like to exchange their role in order to they behave this way:

• MASTER becomes SLAVE
• SLAVE becomes MASTER

I looked for some documentation but I only found information to change the role of a SLAVE to become MASTER but nothing to reverse the sense of the replication (SLAVE becomes MASTER).

Can you please me provide a full step-by-step procedure to do so?

Thank you.