Automatically move to a specific slide in Powerpoint?

Between my suggestions and CharlieRB's I think you have an answer so I'll summarize here.

You can put a copy of Slide 6 right after Slide 2 and have it automatically appear, then proceed with the rest of the slides, eventually landing on Slide 6, the original, again.

Or you could put Slide 6 right after Slide 2, then on Slide 6, add a hyperlink to a custom show composed of slides 3,4,5, and checkmark the option to show and return. You'd land on slide 6, click the link when ready; you'd then be able to view 3,4,5 and when done, PPT would automatically return you to slide 6. You'd want to hide slides 3,4,5 so that when you're done with slide 6, you'd move on to slide 7.

In all cases, I'm using your original numbering, but you'd actually have, in the first example:

Slide 1
Slide 2
Slide 6 (copy)
Slide 3
Slide 4
Slide 5
Slide 6

and in the second example:

Slide 1
Slide 2
Slide 6
Slide 3
Slide 4
Slide 5

The current Office 365 releases of PowerPoint offer another option - Slide Zooms. On the Insert tab, click Zoom > select zoom type

The Zoom feature creates hyperlinked thumbnails of selected slides on a summary slide. Clicking the thumbnail jumps to that part of the presentation. Getting to the end of that part returns you to the summary slide. I organize my presentations into sections, so the Section Zoom has been very helpful in casually moving around the presentation, and is more visually interesting than a text list of hyperlinks.

An alternative method of achieving the end goal is to use the various slide navigation options in presentation view, or re-order the slides as was mentioned. My favourite navigation trick is to type the number of the slide I want, and press ENTER. If I want slide 6, I type 6 ENTER. That's it. So easy.

For a more sophisticated method of branching that doesn't tie you into a specific order of presentation, place an object on the slide and add a hyperlink to the desired slide. Want to go from slide 3 to slide 6? Click the object. Want to go from slide 3 to slide 4? Don't click the image. Place the corresponding return hyperlinked object on the destination slide.

Use any of the above-mentioned strategies (including Custom Shows) to go to your desired next slide.

You can use a custom show.

In PowerPoint 2013 on Windows, select the "Slide Show" tab on the ribbon. Then select "Custom Slide Show" and click "Custom Shows..." to see/edit all shows. A dialog window will pop up and here you can click "New...". Now you can add each slide you want, in whatever order you want, and even use them multiple times. The best part about this is that if you change a slide you used multiple times, each instance is updated (because they're really just the same slide).
_{This capability has been in PowerPoint at least since 2003; the steps to configure it may vary.}

In order to use this custom show, you can select the "Slide Show" tab, then select "Custom Slide Show" and click the custom show you created in the drop down list.

OR

You can make your custom slide show be shown as the default. On the "Slide Show" tab, click on "Set Up Slide Show"; under "Show Slides", click on "Custom Show" and select the custom show that you want. Now, if you click one of the "Start Slide Show" buttons or press F5, PowerPoint will display your selected show.