How can I edit a pre existing component in Altium design?
If you already have the .schlib or .pcblib. You simply open it, then open the Schematic/PCB Library panel in the bottom right. Then you can double click on any part and start editing with the toolbar at the top and properties.

If you do not have the libraries created, you can easily make them by, when in you schematic or PCB design, clicking design -> Make PCB/Schematic Library on the top toolbar.

Hope this helps.