If by this you mean the Ribbon button:
Navigate to Setting --> Customizations --> Customize the System. This will open the default Solution dialog.
- Expand the Entities node to see the list of entities.
- Find Opportunities and expand the list of customizable areas.
- Click on the Message tab.
- Update the Add Opportunity Product message to Add Opportunity Asset.
- Save everything and publish the entity
If this message does not appear in the list of Messages (some system messages do not for some odd reason), you'll have to modify the Label maunally in using the RibbonDiffXml element in the Opportunity Customization file by exporting the Ribbon and the Opportunity
into a solution. If you need to do this, let me know and I can point you to a resource to accomplish this...
--Dodd