My ex is trying to change my daughter's last name without my permission. What are the steps that needs to be taken to stop this.

Find out what court this is going through and file your objection to the court. "She won't let me see..." you can do something about that. File an enforcement.

File an objection with the court, where she files a request for name change, most likely it the one in the local county where she resides. Moreover, file a request for visitations with the court that has jurisdiction have custody and divorce matter. That should also be the one where your daughter resides for more than 6 months.

You do not have to take any steps to stop it. She has to take steps to do it. Your concern and focus should be more on getting court orders made in Florida for custody and your visitation rights, assuming that is the state with child custody jurisdiction. You need family law counsel in Florida to help you and represent you, especially since you live in Texas.