There are 3 methods to convert a lead into a client:
METHOD 1: LEAD - CONVERT CLIENT BUTTON
The easiest way to convert a lead into a client is to do it from the lead section. Click on the button "Convert as client" .
METHOD 2: LEAD STAGE
The last stage of each Opportunity Type is considered the final stage of a lead, the one that converts a lead into a client. When you select the last stage, it will be automatically converted into a client.
METHOD 3: QUOTE MODULE
The other automated way to convert a lead into a client is by creating a quote and convert it into a contract. By doing so, the lead is converted into a client automatically.
NOTE: If you want to know how to create quote check this link : How can I create a quote / estimate? (6 methods)