There are 2 methods to convert a client to a lead:

Method 1. From the CRM listing (Bulk action)

1. From the CRM listing apply the filter see CLIENT LIST "Yes". 

2. Once in the list of clients (not leads) select the clients that you want to convert into a lead.

3. Once selected go to batch action and select the option "Convert to lead".

Method 2. From the lead edition mode

From the "Client" edition mode (Having filtered the listing to see only clients within the CRM), at the bottom, you will see a button  "Convert to lead". Click it.