How do I change what textures I have to paint my landscape after designing my world?

I am new to game design and have been learning as I go when designing my new world. I greatly appreciate any help I can get! I have been trying to change the materials and textures used for painting my landscape but I cannot seem to be able to use any custom textures/paints or change from the default paint textures. Furthermore, The flowers and foliage won’t disappear from the grass areas even when I paint over with sand/dirt/gravel etc. I would rather not generate a new world since I’ve spent some time designing my world layout but it’s starting to seem I may have to go that route.