How do you add trimmer line to Troy Bilt TB539E Weed Eater?

This model trimmer uses .095" diameter line, your local home improvement store will carry replacement line and it will also carry a replacement spool with the line already spooled on it.

This trimmer is a "bump advance" trimmer meaning you bump the head on the ground while it is spinning to advance line out.

What to do to replace the line is place the trimmer on a stable flat surface (like a workbench), raise up the head if the trimmer and unscrew the cap and CAREFULLY remove the spool...you do not want to lose the spring! Take a look at the spool, it will show the direction to spool the line, spool the line then replace the spool by first putting the spring back on the shaft, then put the end of the line through the hole(s) on the side of the outer head, press the spool in place and screw the cap back on.

Of course you can eliminate about 90% of the headaches by buying a pre-spooled head at your local home improvement store...then you follow the directions above and can skip the spooling part. Cheers!