I think OP is asking how to get the old line out since it didn't load evenly and now only one end of it extends from the trimmer head.
The manual is not super detailed on how to do that. But, it does mention to hold the powerload button down "briefly" until the cover releases. (Words to that effect) OP, check page 23, near the bottom, of the manual. It's basically just before how to load the string section.