The build time of a unit can be found in the "Train" Abilities of the various buildings in StarCraft II. Open up the Data Editor (F7) and under "Data Type:" select "Abilities." Search for "Train" and you should see a list of building names followed by the word "- Train."
Select one of these Abilities and in the box on the right, find "Ability - Info +." Double-click on the space on the right to bring up a window that lists all the units that building can train.
Double-click any of those units listed to bring up another window. Find "Info - Time," and modify the value as you see fit. You have now changed the build time of that unit.