add missing step of installing next@latest (#26141)

- Step for installing the latest next version was missing
- Turned "Upgrade React version to latest" to an h3 heading



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.

## Documentation / Examples

- [ ] Make sure the linting passes

docs/upgrading.md

@@ -6,7 +6,7 @@ description: Learn how to upgrade Next.js.
 
 ## Upgrading from version 10 to 11
 
-## Upgrade React version to latest
+### Upgrade React version to latest
 
 Most applications already use the latest version of React, with Next.js 11 the minimum React version has been updated to 17.0.2.
 
@@ -22,6 +22,20 @@ Or using `yarn`:
 yarn add react@latest react-dom@latest
 ```
 
+### Upgrade Next.js version to latest
+
+To upgrade you can run the following command in the terminal:
+
+```
+npm install next@latest
+```
+
+or
+
+```
+yarn add next@latest
+```
+
 ### Webpack 5
 
 Webpack 5 is now the default for all Next.js applications. If you did not have custom webpack configuration your application is already using webpack 5. If you do have custom webpack configuration you can refer to the [Next.js webpack 5 documentation](https://nextjs.org/docs/messages/webpack5) for upgrading guidance.