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 computational material science. I write efficient codes for simulating light-matter interactions at atomic scales. I like to develop Physics, DFT and Machine Learning 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.