Question

Butter is made up of milk, change the voice

Answer 1

We make butter from milk.

Answer 2

"Butter is made up of milk" – Passive Voice

"We make butter from milk" – Active Voice

In the active voice, if the subject is “unimportant or unknown or obvious”, we will omit “by + object.”
In the above sentence “by us” has been omitted.  In Voice, tense won’t change and the verb will be in agreement with the subject.