When opened, the tailgate (boot) is supported by two gas struts.
If the tailgate (boot) won't stay up and falls down, replace struts in pairs.
To replace each strut
Securely support the tailgate open
With a small screwdriver, disengage the connection by flicking the steel springs out of each end of one strut
Pull the strut off the ball joints
Clean and lightly grease the ball joints
Fit the new strut - there is already some grease inside - by pressing firmly onto the ball joint. You should hear a click as the steel clip engages the ball joint.