Just update `|=` the `defaultBranchRef` field to the content of its `name` subfield:
~~~sh
jq '.[].defaultBranchRef |= .name'
~~~
~~~json
[
  {
    "defaultBranchRef": "main",
    "nameWithOwner": "KyleMit/stack-posts"
  },
  {
    "defaultBranchRef": "master",
    "nameWithOwner": "KyleMit/HelloSERN"
  }
]
~~~
[Demo](https://jqplay.org/s/YgodN5agGI)

I'm trying to simplify a nested JSON structure using JQ

### Input

```json
[
  {
    "defaultBranchRef": {
      "name": "main"
    },
    "nameWithOwner": "KyleMit/stack-posts"
  },
  {
    "defaultBranchRef": {
      "name": "master"
    },
    "nameWithOwner": "KyleMit/HelloSERN"
  }
]
```

### Desired Output

```json
[
  {
    "nameWithOwner": "KyleMit/stack-posts",
    "defaultBranchName": "main"
  },
  {
    "nameWithOwner": "KyleMit/HelloSERN",
    "defaultBranchName": "master"
  }
]
```

### Attempted Solution

I can retrieve each of the individual fields at any depth

* [Get `nameWithOwner`](https://jqplay.org/s/6H2F1AkdAO)

    ```js
    [.[]|.nameWithOwner] // ["KyleMit/stack-posts","KyleMit/HelloSERN"]
    ```

* [Get `defaultBranchRef.name`](https://jqplay.org/s/ySBjK4KpsY)

    ```js
    [.[]|.defaultBranchRef.name] // ["main","master"]
    ```

But can't quite get how to combine fields / selectors to return mulitple values