You can do this by overriding the 3D texture of the operator's shirt, using the Customize Shape window:
Click the More Visuals button in the Visuals panel of Properties. Then push the Customize Shape button. Then choose the Tops mesh (his shirt), check Override, and then load a custom 3D texture for the shirt. Here is an example one I made by modifying the file in C:\Program Files\FlexSim 2021\fs3d\Operator\OperatorMale_LowRes\OperatorMale_Top.png
To keep the 3D texture on the shirt you'll need to also copy the Operator_MaleTop.normalmap.png file and put it in the same place as the other image. Make sure it has the same name as the other image too, but ends with .normalmap.png. So in my case I would name it CustomLogoTop.normalmap.png.
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