If you're using WordPress version 5.0 or later, the Classic Editor plugin can be installed to restore the old post editor interface. Here are the steps to install the Classic Editor plugin:
Log in to your WordPress dashboard.
Go to the "Plugins" section in the left-hand menu and click on "Add New".
In the search box, type "Classic Editor" and hit enter.
You should see the Classic Editor plugin in the search results. Click the "Install Now" button next to it.
After the plugin is installed, click on "Activate" to enable the plugin.
Once the plugin is activated, the classic editor interface will be restored.
That's it! You can now use the classic editor to create posts and pages in WordPress.