what mean this word "Marched back up"??

This “back up” is an adverbial phrase (副詞句). It has two adverbs. The first adverb, “back,” conveys the information that she is returning to the place she came from (戻る). The second adverb, “up,” conveys the information that she is doing it with great intensity (力強く).

Compare these sentences.

She walked to the front of the room.
最低限の情報のみ表現。

She marched to the front of the room.
リズミカルに脚を動かしながら、何かの意図を持って動いた様子を表現。

She marched up to the front of the room.
目的地がまるで高い場所にあるかのように、力強く動いた様子を表現。

She marched back up to the front of the room.
彼女が以前にいた場所に戻ったという情報を追加。

ちなみに、目的地がまるで低い場所にあるかのように、気楽に歩くときには、down を使います。

I walked down to my friend’s house.