Remove the screen frame from the window
Lay the frame on a flat surface
Pry out the old spline with a flat tool
Remove the damaged screen mesh
Cut a new piece of screen mesh slightly larger than the frame
Place the new mesh over the frame
Press the mesh into the frame groove with a spline roller
Insert new spline into the groove to secure the mesh
Trim the excess mesh with a utility knife
Reinstall the repaired screen frame into the window