When installing a beta version of the plugin, you cannot install through the WP plugin panel since the plugin is already installed. In order to install it, you have to
Take note of the plugin’s change log (always in the readme.txt file or a release announcement on the site) to determine if there are any database changes. If there are, you’ll need to make note of these if you need to roll back. Database changes are rare, and if at all possible they are limited to major version upgrades (i.e. 3.2 to 3.3) so most of the time moving forward or backward is not a major undertaking.
Here are some methods you can use to install and test a beta version.
Install using WP-CLI (best option)
The best method to install a beta version of the plugin is to use WP CLI. When using WP CLI, enter the following command:
wp plugin update wp-members --version=trunk
There are no database changes in this version, so if you need to roll back, you can use the following command (where the version number matches the version you are rolling back to):
wp plugin update wp-members --version=3.3.6
Manual installation from wordpress.org
Anytime there is a beta release of the plugin, you can get it from the wordpress.org repository by going to https://wordpress.org/plugins/wp-members/advanced/ and selecting “Development Version” from the dropdown selector in the “Advanced Options” section.
Note that doing it this way requires that you transfer the files via FTP. You can either completely replace the existing version (best to deactivate first), or you can install in another directory to toggle back and forth. Either way, it’s best to deactivate the existing version and activate the beta to make sure any update scripts are run.
Install from github
You can always get the latest development version on github. Note that the github version is always the “bleeding edge” update and may include updates (including debugging changes) so at any given time it may differ from the version on wordpress.org as official beta. If you’re familiar with git, you can install directly via command line or git client, or you can download and install manually (similar to installing manually from wordpress.org).