Marlin Adjusting feedrate
You can change the maximum allowable feedrate in Configuration.h, but the actual feedrate that is used isn't determined by your firmware. The feedrate is specified in the G-Code file. A command like

G0 X10.0 Y15.0 Z3.0 F9000


indicates a move to (10,15,3) at a feedrate of 9000 mm/min. If F is not specified, the last used feedrate is used.

You just have to provide the appropriate G-code commands with the feedrate you want in them. There's no reason to modify the firmware to get a different feedrate.