Generally you'd want to let Zotero rename it based on the parent item metadata. For files downloaded from the web that will happen automatically. For a file you drag in, you can trigger that via the item context menu.
If you want more customization options, see the ZotFile plugin.
If you really want to rename it manually, you can click on the attachment item and click the attachment title at the top of the item pane. Alternatively, right-click → Show File, rename the file in your filesystem, double-click on the attachment in Zotero, and use the Locate button in the error dialog to relink it.
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If you want more customization options, see the ZotFile plugin.
If you really want to rename it manually, you can click on the attachment item and click the attachment title at the top of the item pane. Alternatively, right-click → Show File, rename the file in your filesystem, double-click on the attachment in Zotero, and use the Locate button in the error dialog to relink it.