How can you go to undernet 7 in mega man 3 white

Defeat FlamMan's V2, available in a dead-end in Undernet 5. Go to Undernet 6, and go up and left as far as possible. Through trial and error you will eventually get to an isolated path leading up and left, until you hit FlamMan's security cube. It will dissolve, and you will be free to continue on, to the top of the area and thence to Undernet 7.