When would dispair be used in weapon mods, in the rule book it gives out comes for dispair but in what situation would i upgrade the difficulty dice to a red?

Whenever the GM wants.

This could happen because the GM flips a Destiny Point or just because the GM says so, for various reasons.

You could just do it when they hit their 4th mod of a given attachment if you wanted. There aren't that many that even have that number of mods. You could save it for just Destiny Points like progressions mentions and have it occur for reasons known or unknown, maybe the attachment is defective and the players don't know, or whatever works for you.