<p>So I am trying to get the XSD-Schema from this object. When I open the File I just get something like this code (snippet):</p>

<pre><code>¸†2¡Q·2ºyƒeCã2ï…w ïÀ|¼ðAøä[0Ÿ |&gt;‚|ó‘å2²ºFƒ¼Æò1ùàåcj@Þ`ùÐ¸Ì{áÈ;0/|¾ 
 ùÌ'Ÿ„ Á|d½¬¬¯Õ ¯±|l¾…­Œo@Þ`ùÐ¸Ì{áÈ;0/|¾ùÌ'Ÿ„ Á|d½œ¬¯Ó ¯±|h\æ­Œo@Þ`ù¸|
 ßBs¦5–Œ~ôè»­£(™c´“Ç£[yp1:æ'Ã‰c4Jó Uâ˜ÍÇ&lt;h—8^'Ð¯É
</code></pre>

<p>What is this? How can I convert it back? I need to edit the structure manually.
<br>Thank you in advance.</p>

Seems to be your are trying to edit a file with the wrong file editor.
If you have Microsoft® Windows® 8.1, you can print directly using the 3MF format included. Simply set the print options in the 3D Print PropertyManager and print to the 3D printer. A preview of the print bed and the model's location within the print bed lets you modify settings before committing to a 3D print job.
To access the 3D print dialog box and specify print options, click File > 3DPrint.  The print dialog box that is available depends on your installed 3D print driver.
If you need to get the STL you may need to use a file conversion,
here
is a youtube tutorial to makeprintable