Dell.com has the official user manuals for the Dell Optiplex GX260, as of December 2015. The website provides the manuals for free in PDF format.
From the Dell website home page, click on Support, and then click on View All Support. If you are using your Dell Optiplex GX260, click on the Detect Product button. If prompted, download and install the detection software, and then revisit the Detect Product page. Alternatively, you can use the service tag or product ID of your Optiplex GX260 to locate its manuals. Enter the tag or ID in the appropriate text field in the Support page, and then click on the Submit button.
You can also manually browse for the Optiplex GX260. Click on View Products in the support page, and then select the appropriate product type, family and model. From the Optiplex GX260 product page, click on the Manuals tab, and then click on the link for the manual.