Questions & Answers about Мне нужно поменять батарейки в часах.
Because Russian often uses the pattern [dative pronoun] + нужно + infinitive to mean someone needs to do something.
So:
- мне = to me / for me
- нужно = necessary / needed
- поменять = to change
Literally, the sentence is something like To me, it is necessary to change...
This is a very common Russian way to express need.
Compare:
- Мне нужно поменять батарейки. = I need to change the batteries.
- Я должен поменять батарейки. = I must / am supposed to change the batteries.
Я должен can sound more like obligation, duty, or external expectation.
Мне нужно is usually a more neutral I need to.