how do i access my text messages from my back up on icloud

If you aren't syncing Messages using iCloud, the only way to access the messages in a backup is to restore the backup. Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup to see the contents and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.    *

ASC YouTube for how to restore an iPhone, iPad, or iPod touch from an iCloud backup

If you use messages in iCloud, there is no backup kept. iCloud simply acts to sync messages between your devices.