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Generally you would start a project in the Freelance 3D Projects. However, it depend on the license of the STL you are asking about. If it's your design, then you would explain what you wanted to get modified, but if you purchased the model and if the license prevents you from changing it, it would be illegal for anyone to change it without express permission from the original artist. It also depends on what and how you want it modified.
Thank you
