what does "back up" mean in this context:

First, you need to understand that "scale up" is a phrasal verb: a verb followed by a particle (usually an adverb or a preposition) that combine to have a different meaning than the verb by itself or the literal meaning of the verb plus the particle. To "Scale up" means to make something larger in some way.

In your sentence, back is an adverb modifying "scale up", using sense 2a from M-W's definition:

to, toward, or in a place from which a person or thing came

• She left home and never went back.
• put the book back
  _{from m-w.com}

The placement of the adverb between the two parts of the phrasal verb (an "infix", rather than a prefix or a suffix) can definitely make this hard to parse, but the intent is clear to me that earlier, the habit being discussed has been "scaled down", and now the goal is to "scale it up", in a return to its original form.

This is especially confusing when you consider that you can have:

• scale down: to reduce
• scale back: to reduce
• scale up: to increase
• scale back up: to increase to a previous level
• scale back down: to reduce to a previous level