Creating a 3D Model with Tinkercad - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 136

Figure 7.11 Make a copy of the Round Roof object for other side of medallion.
You could use the rotate feature to spin the Round Roof copy, but an easier method
is to select the copy, click the Adjust drop-down menu, and select Mirror. Click
the arrow indicated in Figure 7.12 .
Figure 7.12 Flip the object so it is a mirror of the original.