To change the Title of an Action Bar which is basically the title of an activity:

• Open the AndroidManifest.xml file and edit the label of an activity as shown:
  

That’s it!!!
I hope it was helpful.
If you have any query or doubt leave them in the comments section down below.

Manas Sharma

PhD researcher at Friedrich-Schiller University Jena, Germany. I'm a physicist specializing in theoretical, computational and experimental condensed matter physics. I like to develop Physics related apps and softwares from time to time. Can code in most of the popular languages. Like to share my knowledge in Physics and applications using this Blog and a YouTube channel.

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