If it's in the EElib, double-left-click on it to open it in the Editor.
Otherwise in the Libraries tool, select it and click the Edit button.
In both cases as long as the **only** thing you change is the appearance of the Schematic Symbol and you don't change the Symbol to Footprint pin mapping, Footprint, Supplier and the Supplier part number, then you shouldn't interfere with the JLCPCB PCBA process.
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