For everyone, you can find the links to download the RC1 here:
Originally posted by @camila in Release 2.6.1 #1460
Obviously this is worth a look and may be the opportunity for the more adventurous user to identify issues for your OS environment at your convencience. However, be warned this is ALPHA BETA phase apparently and funny things can happen.
Please think first, act second only and always have the protection of your data in mind. 
Reporting issues
- If you find any bugs or have any suggestion for improvement, please file an issue at https://github.com/nextcloud/
clientdesktop/issues. Do not contact the authors directly by mail, as this increases the chance of your report being lost.
Mac OS X / 
macOS trials
Please consider the availability and security of the NC server and the data in your NC cloud storage should matter most. It may be advisable to use a more safer environment and not a productive cloud install if possible.
Please note the availabilty of the NC Issue Template App which could help in producing more detailed reports when issues are found.
However, the NC 17 version is not supported by the NC Issue Template App yet:
Before starting some of the more ambitious trials consider to make more extensive logs available by starting the NC desktop app from the command-line (CLI) such as:
Use the Apple Terminal for CLI input and a special GUI window will open:
$ /Applications/nextcloud.app/Contents/MacOS/nextcloud --logwindow
NOTE: Keep the Apple Terminal window open as long as you are using the NC desktop app called by the a.m. command.
Naturally, one may stop i.e. ‘quit’ the installed standard NC desktop app to intentionally start a specific NC desktop trial app version instead of the a.m. standard app by help of the a.m. command.
Ref to the documents acc. to your locale at your convenience as:
EN locale
- Nextcloud Client 2.5 Manual - Appendix C: Troubleshooting
- Nextcloud - NC 16 Docs » Issues and troubleshooting
- Nextcloud - NC 16 Nextcloud developer documentation
- Apple support How to force an app to quit on your Mac
- Apple support How to use Activity Monitor on your Mac
- Apple support Terminal User Guide
- The Programming Historian - Introduction to the Bash Command Line
- Apple support Find out which macOS your Mac is using
- Apple support Use zsh as the default shell on your Mac
ES locale
- The Programming Historian - Introducción a la línea de comandos en Bash
- Soporte técnico de Apple - Manual del usuario de Terminal
DE locale
- Apple Support: Terminal-Benutzerhandbuch
- heise Mac&i: macOS-Terminal – Das kleine Einmaleins der Kommandozeile
- Apple Support: Das Schließen einer App auf Ihrem Mac erzwingen
- Apple Support: Die Aktivitätsanzeige auf dem Mac verwenden
- Apple Support: Das Betriebssystem Ihres Mac ermitteln
- Apple Support: zsh als Standard-Shell auf dem Mac verwenden
or any language which suites you more.
Furthermore, when using the Nextcloud desktop v2.6.1-rc1 app beware of some of the ‘funny’ things which happended during my trials:
NOTE: When you’re working in Terminal, you don’t have a Trash Can to which deleted files are moved pending ultimate disposal. Delete it, and it’s gone. In general, UN*X (like most mature operating systems at CLI level and for good reasons) has no Undo function.
Hope this helps.
Happy hacking. 
Reporting issues
Mac 10.10+ legacy
) or a little ACK to one or more of my comments (i.e. click on the heart icon 
) would show you are satisfied. This could be a kind gesture and would motivate me like authors of other advice to continue in lending a hand freely…