Do you mean how to change a PCB package that is used in a PCB Module that someone has uploaded?
If you mean in a PCB of your own design then just delete the old one and after doing a SHIFT+F search for the replacement, click on `Place` in the found item.
If you're doing a PCB from a schematic then update the package in the schematic then do `Import changes...` back in the PCB editor.
There's also a note about doing PCB design in the **Tips and Skills** section.
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