This method will guide you to save the attachment you will rename as another copy with new name, and then reattach the new copy in the email. Please do as follows:

1. In the email, right click the attachment you will rename, and select Save As from the context menu. See screenshot:

2. In the opening Save Attachment dialog box, please (see screenshot below):
(1) Open the destination folder you will save the new copy of selected attachment to;
(2) Type the new name that you will change selected attachment’s name to in the File name box;
(3) Click the Save button.

3. Now you return to the message window. Please click the attachment you will rename, and press the Delete key to remove it.

4. Click Insert > Attach File > Browse This PC (or Insert > Attach File) to reattach the new copy of the specified attachment.

5. Now the Insert File dialog box comes out. Please do as following screenshot shown:
(1) Open the folder where you saved the new copy of the specified attachment just now;
(2) Click to select the new copy of attachment;
(3) Click the Insert button.

And now the new copy of the specified attachment has been attached with new name.

6. Please compose the email and send it.

If you need to rename an attached file in a new composing email or a replying/forwarding email in Outlook, you can apply the Rename (Attachment) of Kutools for Outlook to get it done easily.

1. After inserting attachments in a new composing email or a replying/forwarding email, select the attachment you need to rename, and click Kutools > Rename.

2. In the popping out Rename dialog, please type in a new name, and click the OK button.

Then you will see the specified attached file is renamed.

With the Kutools for Outlook, you can also rename multiple or all attachments in bulk in a new composing email or replying/forwarding email in Outlook.

1. In a new composing or replying/forwarding email message window, click Kutools > Rename All to enable the feature.

2. In the popping out Attachments Rename dialog, please double click an attachment name to make it editable, then type in a new name. If necessary, please repeat this step to rename other attachments.

3. Click the OK button.

Then you will see the attachments are renamed in bulk. See screenshot: