Adding or modifying opportunity types is very simple, just follow these steps:
1. Go to Setup / System Values
2. Locate the "Opportunity Type CRM" section
3. Click on the "+" button to add a new opportunity type or on the icon marked with a trash can to delete it. You can also edit a current opportunity by clicking directly on the Opportunity name you want to modify.
IMPORTANT!! Keep in mind that modifying and deleting and opportunity type will affect all current leads using that opportunity type.