Add/Edit and Quick actions (especially accounts)

Add/Edit and Quick actions (especially accounts)

at Leads you can see underneath Quick actions: 
Add a new task,
Add a new event, 
Add a new call, 
Send mail

at Contacts you can see underneath Quick actions:
Add a new potential, 
Add a new task, 
Add a new event, 
Add a new call, 
Send mail 


at Accounts you can see underneath Quick actions:
Add a new task
Add a new event
Add a new call

My question is : How do I Add/Edit Quick actions ? For example how do I add 'Send mail' underneath the Quick actions of Accounts??