You'll need to be more specific in what you are trying to achieve.
But let me guess, you are trying to aplly an buff to an spell, right?
Then you need to go to object editor (F6) under the abilities section, select the ability you want to change the buff. Then find the field titled "Stats -Buffs" and change it as your desire. You can make or copy custom buffs under the Buffs section in the object editor.