How to add repository with plugin I need to edit in composer.json?

To add a repository with a plugin that you need to edit in your composer.json file, follow these steps:

First, you need to locate the plugin you want to add to your composer.json file. You can search for it on Packagist, the official repository for Composer packages. Once you have found the plugin, note down its name and the repository URL, if it's not available on Packagist.
Open your project's root directory in your terminal or command prompt and run the following command to initialize Composer if you haven't already:
```
composer init
```
Follow the prompts to set up your project's basic details.
If your project already has a composer.json file, you can edit it directly. If not, Composer will create one for you. Add the repository to your composer.json file under the repositories key. Here's an example of how to add a repository for a plugin called myplugin that is hosted on GitHub:
```
{
    "name": "myproject/myproject",
    "description": "My project description",
    "license": "MIT",
    "require": {
        "myplugin/myplugin": "^1.0"
    },
    "repositories": [
        {
            "type": "git",
            "url": "https://github.com/username/myplugin.git"
        }
    ]
}
```
Replace username with the GitHub username or organization name that owns the plugin repository.
Save the composer.json file and run the following command to update your project's dependencies:
```
composer update myplugin
```
This command will download the plugin and its dependencies into your project.
Once the plugin is installed, you can edit its files by locating them in your project's vendor directory. For example, if the plugin has a file called MyPlugin.php, you can find it in the vendor/myplugin directory.
Make the necessary changes to the plugin file and save it. If you need to commit the changes back to the plugin repository, you will need to fork the repository, make the changes in your forked copy, and submit a pull request to the original repository.
Finally, run the following command to update your project's autoload files:
```
composer dump-autoload
```
This command will update your project's autoload files to include the changes you made to the plugin file.