The latest version of WordPress supported by MemberMouse is:
Downgrading to a Previous Version of WordPress
If you've accidentally upgraded to a version of WordPress that is higher than the version we currently support, you can downgrade to the previous version of WordPress by following these instructions:
WordPress Release Archive
Restoring Site from a Backup
If issues were caused by upgrading to an unsupported version of WordPress and the issues persist after downgrading you'll need to restore your site to a latest backup prior to upgrading. If you're unsure about how to do this, your hosting provider should be able to provide some assistance.