How do i lower the rear shock on a 450 2004 crf

I would buy a new aftermarket one from a company such as Maxton, Hagon or one of the big suspension companies such as Showa or Ohlins. When you try to replace the fluid you will also need to find out where it is leaking from, and then fix it.

Put the bike on a stand supporting the bottom of the frame. Remove the seat and sidepanels. Loosen the clamp on the carb to airbox boot at the rear of the carb. Loosen the muffler clamp, remove muffler mounting bolts and pull muffler. Remove the 4 rear subframe bolts. Remove the rear subframe (leave airbox and rear fender assembled in it). You are now staring at a completely exposed rear shock. Remove the shock mount bolts and remove the shock. Measure the spring adjusted length for a future starting point when the spring is put back on. Loosen both spring pre-load adjuster spanner nuts all the way. Helps if you clean the dirt off the threads and use a little lube. Pull the spring towards the spanner nuts and remove the clip ring at the opposite end spring holder. that's it.
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Call around some local dealers or parts places, especially ones that are into racing. They should be set up to ship your shock out to a good suspension company. "Factory Connection, C-Cycle, W.E.R, to name a few"

Shock Absorber -The rear already has air shocks on the 1999 Cadillac Deville: REMOVAL & INSTALLATION Rear

1. Before servicing the vehicle, refer to the precautions in the beginning of this section.
2. Remove or disconnect the following:
   • Rear wheel Fig. 1: Exploded view of the rear shock absorber mounting
     
     
   • Negative battery cable
   • Shock absorber electrical connector from the rear suspension support, If equipped
3. Support the lower control arm with a jack to relieve the tension on the shock absorber.
4. Remove or disconnect the following:
   • Lower shock absorber mounting bolt and nut
   • Upper mounting nut, retainer, and insulator
   • Shock absorber compress by hand and remove through the upper control arm
   To install:
   
5. Position the top of the shock absorber with the insulator attached into the suspension support.
6. Install or connect the following:
   • Upper shock insulator, retainer and nut. Torque the nut to 55 ft. lbs. (74 Nm).
   • Shock absorber lower mounting nut/bolt. Torque the nut to 75 ft. lbs. (102 Nm).
   • Shock absorber electrical connector to the rear suspension support, if equipped
   • Rear wheel
   • Negative battery cable

Put the bike on a stand supporting the bottom of the frame. Remove the seat and sidepanels. Loosen the clamp on the carb to airbox boot at the rear of the carb. Loosen the muffler clamp, remove muffler mounting bolts and pull muffler. Remove the 4 rear subframe bolts. Remove the rear subframe (leave airbox and rear fender assembled in it). You are now staring at a completely exposed rear shock. Remove the shock mount bolts and remove the shock. Measure the spring adjusted length for a future starting point when the spring is put back on. Loosen both spring pre-load adjuster spanner nuts all the way. Helps if you clean the dirt off the threads and use a little lube. Pull the spring towards the spanner nuts and remove the clip ring at the opposite end spring holder. that's it.