d8 damage dagger

We found some daggers in our campaign, The only difference with them, is that do 1d8 damage. I have created one and set the "modifier" to reflect this, but in the inventory, it still says d4 and it does not save to save for me. 

It's not currently possible to change the base damage on a homebrew magic weapon.

What you can do is start with any of the weapons that have a d8 damage die already, and use homebrew modifiers to change its properties so it matches those of a dagger.