The Replace Object button will replace the selected object with another object from the database, or a randomly generated object.
Clicking the Replace Object button will open a menu similar to the Add Tool's menu, providing a searchable and categorized list of all available objects from the Universe Sandbox database, plus randomly generated objects such as Random Rocky Planet and Random Gas Giant. After the user selects an object and then clicks the Replace button, the new object will replace the old object in the simulation, with the same Velocity Vector as the previous object.
Location
The Replace Object button can be found in the Actions section of the Overview tab of an object's properties panel, as well as at the top of the Misc section of the Actions tab.