You will need to work in a well ventilated area. Before beginning spread a drop cloth over the work area to make cleanup easier when you're finished. There are subtle differences between different string trimmer models but all the general procedures will be outlined but remember to read the documentation that came with your strimmer for maintenance. Before starting make sure all moving parts have come to a stop and if the strimmer is gasoline make sure the spark plug wire is removed from the spark plug and make sure the electric trimmers are removed from their power source.
Replacing the trimmer Line
Regardless of the type of trimmer at some point the strimmer line will need to be replaced and there are several different methods for doing this, depending on whether your trimmer is single or dual line and whether or not you use pre cut pieces of line. Use a pre wound spool or wind the line yourself. Follow the manufacturer's instruction for your model and use the specified trimmer line. While replacing the line it is a good idea to clean the cutting shield and area around the head. Also inspect the area for damage and wear. You may be able to fix some of the possible issues yourself, but some will require professional attention.