Use lower-pressure gas
Install a lower-power nozzle valve
Install a weaker recoil spring
Install a lighter hammer spring
Reduce valve dwell if your trigger unit allows it
Shorten the gas route or restrict the gas flow with a proper NPAS-style adjustment if compatible
Install a shorter inner barrel
Use heavier BBs if you want lower muzzle velocity measured in fps
Check for air leaks and fix them if present
Have a qualified airsoft technician perform the modification