I am 12 and I was wondering if I could pick to stay with my mom or dad?

First its not a good idea to piggyback your question on another thread. This can lead to confusion. So I've moved your question to its own thread. However, if you read the thread you posted to your question was already answered.

You can't pick, it up to the court to decide. The judge might listen to your preference depending on local laws, but the judge will decide based on what they feel is best for you.

No you can not pick. Some courts ( but not all) may allow a 12 year old to testify in court as to what they would prefer. But the court will still rule according to what is in the best interest of the child.

And to even get to court the non custodial parent will have to file in court to modify the current custody agreement.