Ryobi Trimmer How do I get the trimmer line holder cap off my Gasoline powered Ryobi Trimmer, to put a new line on and how do I thread this line through the two holes coming out of the hub

To install string in the Ryobi you do not have to disassemble anything: Line up the two arrows on the inner and outer reel, remove any excess string, cut two pieces of string about 10 feet, feed line in the side holes, secure line in hole where it comes out the bottom (it only takes about 1/4in. to do so), pull strings tight, then turn the inner reel counter clockwise to wind it. If it sticks a little just pull line a little.
To remove the inner spool in order to remove line that has broken inside, to clean the mechanism, or to perform other maintenance, grasp the outer reel and unscrew the bump knob counter-clockwise. Mine is often very tight and requires pliers to get enough leverage. While you are in there, clean the inner reel and the inside of the outer reel to improve feed performance. The shaft rarely needs cleaning. Do not lubricate. After re-installing the reel, add string (0.080") as described above.