Here is the standard procedure per the Factory Service Manual...
STANDARD PROCEDURE - FUEL SYSTEM PRESSURE RELEASE PROCEDURE
1. Remove Fuel Pump relay from Power Distribution Center (PDC). For location of relay, refer to label on underside
of PDC cover.
2. Start and run engine until it stalls.
3. Attempt restarting engine until it will no longer run.
4. Turn ignition key to OFF position.
5. Return fuel pump relay to PDC.
6. One or more Diagnostic Trouble Codes (DTC’s) may have been stored in PCM memory due to fuel pump relay
removal. The scan tool must be used to erase a DTC.
If you haven't done that first, then the alternative is to remove the rails with the injectors in place and then try to disconnect one injector and contain the pressure bleed. Best bet is to wrap with a heavy towel when you do it to soak up the fuel. It will be messy.
If you are replacing the injectors, you can use a pointed tool to push in the injector ball on one and bleed the pressure that way. However, it will likely destroy that injector.