Learn Swedish: Sanningen kan ändra ens liv. - The truth can change one's life.

Ens is the possessive form of the generic pronoun man (which means 'one' or 'a person' in a general sense). When you make a broad statement about 'one's' life, you use ens.

Could we use sitt instead of ens here?

No, because the reflexive pronouns sin/sitt/sina must point back to the subject of the sentence. The subject here is sanningen (the truth). Since the truth isn't changing its own life, we cannot use sitt. We must use the general word ens.

Why does the sentence use ändra instead of byta for 'change'?

Ändra means to alter or modify something. Byta means to swap or exchange one thing for another (like changing clothes or swapping seats). Since the truth alters your life rather than trading it for a completely different one, ändra is the right choice.

Is liv singular or plural in this sentence?

It means the singular 'life' here. However, because ett liv is an ett-word ending in a consonant, its plural form is exactly the same: liv (lives). You have to rely on context to know it is singular here.