Active verb mengubah is derived from the root ubah “to change” and followed by an active prefix me–.

We have this rule, once found on the ancient tablet of Indonesian grammar commandments:

If thou wouldst add prefix me- to any word starting with a vowel, you shalt change that me- into meng-.

Since ubah starts with a vowel, you must change the prefix me- into meng-, and there you have mengubah. Not meubah or merubah.

Stop harassing the fox!

You see that rubah guy up there?
Pissed rubah

Visit this site to learn more about how to use prefix me-! No complicated explanation; just a colorful chart that will help you learn.

Image credit: Wikipedia/David Monniaux