Hi Keyboard Master,
Unfortunately, Yamaha has omitted some important features from the DGX-670 (which is comparable to the PSR-SX600 in terms of operation). Among other things, there is no Step Edit display here in the Song Creator.
In order to edit individual MIDI events, e.g. to change the velocity values, you would have to load the relevant MIDI file into another keyboard (e.g. S770/970/SX700/900/Genos/Tyros) or edit the file on the computer, e.g. in "MixMaster".
Best regards,
Chris