There is a potential source for confusion here.
A symbol (which is placed to a schematic) is an abstract representation of a part. If you use a symbol for a specific part, it would be best to choose a symbol which has the same pin numbers as the datasheet. You will find generic symbols for mosfets in the Device symbol library, Q_NMOS… and Q_PMOS… There you can see the letters SGD etc. I don’t like how KiCad deals with this, but the last letters describe the pin order. So, if you have a part (reading its datasheet) with Source=pin1, Gate=pin2 and Drain=pin3, you will choose the …SGD variant of the symbol.
Now you can choose the footprint for it – I think you already know how to connect a footprint to a symbol. For example Package_TO_SOT_THT:TO-220-3_Vertical. But you have to be very careful and compare the pad order with the physical part pin order, again reading the datasheet. Notice that a footprint has pads, not pins. Symbols have pins, and physical packages (parts, components) have pins. KiCad doesn’t have parts or components, it has symbols and footprints.
If I understood you correctly, you want to change the pad numbers of the footprint because it has a wrong pad order (wrong pad numbers compared to the datasheet) but don’t know how to do it to get the best results.
You should create personal libraries. If you have parts which you will use in more than one design, I suggest you create custom symbol libraries and custom footprint libraries. Then save the footprint connection in the custom symbol, name the symbol with the actual part name, and the next time you want to use that part, just add the symbol to the schematic and the footprint connection is already there.
And how to create those libraries? You should see for example (Start Here) Frequently Asked Questions and read under Symbols and Footprints “Creating a new … library”. Make the Global so that you can use them in all projects. Now you can select the new library in the library editor’s library list. You can create a new symbol or footprint – depending on in which editor you are – from File → New Symbol/Footprint. But you can also navigate to an existing library and use the context menu (right button menu) on top of an existing symbol/footprint. There you have Copy. Then navigate again to your new personal library and use the context menu → Paste on it. Now you should have the symbol or the footprint in your personal library, and you can modify it.