XrmToolbox is pretty good for quick JavaScript updates (and other things!). Download, install, and connect it to the environment with the script you want to edit.

Install WebResources Manager from the Tool Library. Open it from the shortcut next to the tool's rightmost name or the Tools tab. Then:

• Open the CRM dropdown menu from the upper-left corner.
• Select "Load Web resources from a specific solution".
• Select the solution with your JavaScript file.
• Uncheck "Load all web resources from selected solution" and click OK.
• Uncheck "All", check "JS" and click OK.

All loaded files will appear in the tool's Webresources Explorer, where you can open them directly.

I recommend editing the script in a code editor, though. I use VS Code for this. When I'm ready with my changes, I copy them from VS and paste them into the WebResources Manager file editor, and then I File -> Update and Publish (or Ctrl + U).

If you work with someone else and think they've applied changes after you've opened WebResources Manager, use Get Latest. It'll fetch the newest version of the file from your solution.

This is a heads-up for a small platform quirk(?)—sometimes, the updated script doesn't load, even after a hard refresh (Ctrl + Shift + R). Hard refreshing again seems to work most of the time. Clearing the cache is the most consistent method I've found (dev console open -> right click on refresh -> "Empty cache and hard refresh").

Not sure what to recommend for a local development environment. I haven't used hot reload for scripts.