How to adjust print speed?

The best place to do it is the slicer. Are you slicing in the Web interface or are you uploading GCode to OctoPrint?

You can tweak the speeds in the OctoPrint UI, but really the slicer is the correct place to do that. Which slicer are you using?

 

OK, I'll let someone else comment on that since I have no experience with Cura embedded slicer.
The OctoPrint embedded Cura docs show that the embedded slicer uses CuraEngine Legacy up to and including Cura 15.04.x profiles.
So you should be able to create and upload a profile (but you will need to use the much older 15.x version of Cura). There are download links for that at this page as well as a discussion about it:

https://plugins.octoprint.org/plugins/curalegacy/

as odd as it seems the older Cura has a much higher version number

 

Note that a lot of that linked page is moot since it talks about compling/building OctoPrint -- just the parts under
Using Cura Profiles

are relevant to your question

 

The default Cura Engine for Robo is very old. The best way to access all the parameters for controlling the print job is to use a desktop / laptop version of Cura ( current version is 4.5.0 ) Then send the generated G-code to the Octoprint interface of the R2. This approach insures that you are using the latest Cura Engine and interface.
The R2 FAQ has instruction on how to interface the desktop Cura to the Octoprint interface, link to the latest version of Cura and other good information.

 

The post at the top of the list for this forum labeled "R2 FAQ"


http://community.robo3d.com/index.php?threads/r2-faq.23706/