How can i craete/apply a new unit texture?

[Epistolary Richard]

Ah, I think you're talking about the MipMaps. Yes, they are used at different distances, but you don't want to display them when you're editing the model. When you save it, ensure you generate the MipMaps as well - as it says in the guide I linked to above - they'll be automatically created from the larger version you edited.