Use a text editor such as Notepad, TextEdit, VS Code, Sublime Text, or Atom
Use a Markdown editor such as Typora, Obsidian, Mark Text, or Typora
Open it in a web browser with a Markdown preview extension
Open it in GitHub, GitLab, or another code hosting platform that renders Markdown
Use a file manager and choose Open With to select a compatible app
Rename the file to .txt if you only need to view the raw text
Use an online Markdown viewer or editor
Open it in a code editor and switch to preview mode