How do I change spell tooltips ingame

I have changed how some abilities work and I want to update their tooltips ingame. How can it be done, though? Do I need to create a new .mpq file and how? If not, how can I accomplish it?

Client editing is not allowed here.

I think there must be a way to do it by core, in the priest’s shield I had that need and I did not know how to do it, the tooltip had to say the amount of damage that was remain to absorb the shield (retail), however, it only showed the total amount was going to absorb.

Yes, add-ons (the ui) can edit tooltips. If you inspect it you can see it is showing the charges left as it were, similar to other absorbs.

But you cannot add new spell ids, item ids, etc and their trappings without modifying client files .. which is against the community rules.

Supplying an add-on in a mpq (blizz like) would also be a modification.