I wanted to change the cooldown of Shield Wall, from what I've figured out, this is what I had to do:
I opened Spell.dbc, seareched for Shield Wall(ID: 871),(according to
this, cooldown is represented in column 37). Column 37 had 124. So, I opened SpellCooldowns.dbc and searched for 124 and changed Column 1 from 300000 to 2. After all this, I restarted the server and no change was made.
I don't know if what I did was good or not. That's what I understood I had to do.