By default every texture is low quality in the viewport:

To show detailed textures, enable “Match bitmap size as closely as possible“:

Every new texture you’ll add will be showed in hi quality.
About the texture already assigned: go to the material editor > bitmap > click “RELOAD”

Here how the texture will look in your viewport:

Note:
By default you can’t see detailed textures because default setting is done to preverse as more memory as possible.