Addressing merge conflicts
If your changes have merge conflicts with the base branch, you must address the merge conflicts before you can merge your pull request's changes.
About merge conflicts→
Merge conflicts happen when you merge branches that have competing commits, and Git needs your help to decide which changes to incorporate in the final merge.
Resolving a merge conflict on GitHub→
You can resolve simple merge conflicts that involve competing line changes on GitHub, using the conflict editor.
Resolving a merge conflict using the command line→
You can resolve merge conflicts using the command line and a text editor.