How can I change picture order after uploading a model ?

Please go through this tutorial.
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Step 1:
If you are in the old version of library like this
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Step 2:
Then go to the image which you want to take it to first and tap on set as Defaul Rendering.
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Step 3:
If you are in New version library like this
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Step 4:
Then go to the image and by drop-down Menu click on Set as default Rendering.