Hi,
This explains about how to convert lead in apex.
Reference:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_dml_examples_convertlead.htm
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_dml_convertLead.htm
