Using open source code in my final year project
It is not plagiarism as long as you clearly write something to the effect of "I downloaded this code from this github" in your paper and cite the paper in question. Plagiarism would be to pretend that you wrote all the code from scratch.

(On a side note, the code on github is most likely under an open source license, but you should double-check this before you upload your modified code to github. If there is no license present in the project that explicitly allows it, it would theoretically be a copyright violation to publish your modified code. This doesn't affect the paper, which you can publish even if you can't make your code public.)