I think you are actually talking about the object on the screen.
You would have to open Architect.
Go to Manage Plugins
Change the Plugin type to Form
Find the Contact Details Form under the Contact Family
Open this form and highlight the Spouse Field and right click....chooose properties.
In the Caption Property....change the caption to whatever you want.
Save your Form.
But, remember this does not change the field in the database, when you are creating queries, it will still be the spouse field.
If you need assistance, I would contact your Business Partner.
Thanks
Dale