To open the weed eater head to replace the trimmer line, first, locate the tabs on the side of the head. Press and hold these tabs while twisting the head counterclockwise to unlock it. Once unlocked, carefully remove the head to access the spool and replace the trimmer line.
If this is in fact an Echo SRM 210, then a manual is linked below.
Nylon
The Weed Eater Electric Trimmer is one of the easiest to use
To load weed eater line on a Craftsman trimmer, first, turn off the trimmer and unplug it if it's electric. Remove the spool from the head by pressing the tabs and pulling it out. Cut a length of trimmer line (typically around 10-15 feet), then insert one end into the spool's eyelet and wind the line in the direction indicated by the arrow on the spool. Finally, replace the spool back into the trimmer head and pull the line to ensure it feeds properly.
A line trimmer, also known as a string trimmer, is used to trim grass or weeds. The piece of equipment also goes by the name of a weed eater, or weed whacker.
The manufacturers instructions should include the blade size for your weed eater lawn mower.
http://lawnandgarden.manualsonline.com/manuals/mfg/weed_eater/545117526.html?idRes=15826863 This link should let you obtain the manual for detailed instructions with photos.
To restring a Ryobi Expand-It weed eater, first remove the spool from the trimmer head. Cut a length of new trimmer line, thread it through the eyelet holes on the spool, and wind it evenly in the direction indicated on the spool. Reattach the spool to the trimmer head and trim any excess line.
The best type of weed eater trimmer line for cutting through tough vegetation is a heavy-duty, twisted or serrated line with a thicker diameter. These types of trimmer lines are more durable and can easily cut through dense and tough vegetation.
0.080" per 1993 manual
The best type of string to use for a Kobalt weed eater is a high-quality, durable trimmer line that is compatible with the specific model of the weed eater.