1. Go to Settings
2. Go to Customizations
3. Click Customization the System
4. Click Entities and expand the Lead entity
5. Click on 1:N Relationships
6. If you want to map from Lead to Opportunity, look for opportunity_originating_lead (pictured). If you want to change Account or Contact, look for account_originating_lead and contact_originating_lead, respectively.
7. Find the Mappings section in the top left hand corner of the window that pops up and click Mappings.
8. The following screen pops up
9. Choose the field from the Source (Lead) and map it to a field in the Target (in this case Entity).
10. Click OK
11. Repeat for other fields