Rake is the angle of the steering head and it can be increased by lowering the rear suspension or lifting the front.
The rear can be lowered by reducing the spring preload, replacing the suspension links or fitting a shorter shock or spring, the front by increasing preload, moving the yokes up the fork on some models where as standard the forks protrude from the top yoke.
You could also fit longer forks.
For a radical change such as seen on some choppers, the frame is modified by tilting the head stock, which requires cutting and welding.
Please be aware that motorcycle suspension is quite complex, and alterations can adversely affect the handling.
Increasing the rake will slow the steering.