1. Delete custom field in SFDC and remove from SFDC lead/contact layout
2. Run Step 3 of the CRM Configuration Wizard (Import Metadata) immediately after deleting/remove custom field
Renaming a Custom Field in Salesforce.com
Renaming a custom field in Salesforce will not affect the integration between Lead Liaison and Salesforce. If the name of the field identifier (some_field_name__c) is changed this will remove the field in Lead Liaison and create a new one. Once a custom field is renamed in Salesforce we suggest renaming the Lead Liaison custom field for parity.
Adding a Custom Field in Salesforce.com