If you're looking to downscale footage (as opposed to upscaling it), you'll need to choose 'Bicubic Interpolation' under Filters > Scaler once you've entered the Process Tool.
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After you've chosen Bicubic Interpolation, you'll have to choose the Custom option and enter the resolution you want to downscale to.

P.S. If you run into difficulties or have any questions, please reach out to us here or at help@pixop.com — happy to help 🤓