Yes, you can change your NanoTag anytime. However, you can't use NanoTags that has been used by other users. You can change it by following the steps below:
1. Go to your profile on the top left corner of the home page.
2. Tap on 'Edit Profile'
3. Tap edit under the 'Nanotag' section.
Congratulations! You have changed your Nanotag.