Each brand may do it differently. Your safest bet is to read the instruction manual. If for some reason you can't do that, try googling the product name and perhaps with the word "manual" in the search.
In my experience the bottom part of the weed eater can be unscrewed. Unplug the appliance first. After you have detached the device keeping the spool of string in the weed eater, remove the old spool and insert the new one. Pay close attention to the old spool as you remove it in case the process of inserting the new spool is more complicated than it first appears. After the new spool is in the applicance, refasten the bottom part and ensure it is secure. Plug the appliance back in and give it a careful test, just in case.