You can easily find video instructions on the Internet. There are several ways to change the string. The first thing you need to do is to string the trimmer from the outlet. After that:
Lay down the trimmer and reach the spool after rotation;
Take out the cover of the spool after depressing tabs from both side of the cover;
Take away the spool;
Clean the trimmer’s underside;
Put a new spool inside;
Replace the cover of the spool;
Close the trimmer and reconnect the battery.