Welcome Sam,
I'm sure others who regularly upgrade their audio will chime in but here's my 2 cents worth.
Most door trims are secured the same way and there would be heaps of videos on youtube including Hyundai models and even some i30 ones I would guess showing how to remove them so you can access the speakers. Generally there are couple of screws holding the trim around the inside door handle. Remove those, then carefully use a medium flat screwdriver or trim tool to carefully prise the bottom of the door trim away from the door frame (usually held by push clips) keep your eye out for any screws though just in case. Then lift the trim away from the door frame enough to clear the interior handle and the top of the door trim can be carefully slid out from the window gully. To remove the trim altogether you will need to disconnect the Electric window and mirror connectors Etc..
Watch some Youtube videos to pick up the finer points.