How to add simple resource pack stuff?

Sorry for a simple question but I've looked through the wiki and cant seem to find it.

How can I simply add a diamond sword texture to replace the standard diamond sword?

I want to replace all the vanilla tools and weapons with custom art but can only find stuff for adding new items.

Sorry again if this is a simple thing.

What you can do is find a resource pack that does just that, and then follow the steps on the wiki to merge the resource packs together.