Redirection provides many powerful options and it is possible to accidentally stop your site working, or prevent yourself from being able to log in.
When this happens you can disable Redirection by creating this empty file in your Redirection directory (wp-content/plugins/redirection):
redirection-disable.txt
The plugin will continue to run, but all redirects will be disabled. You should now be able to log in and make your modifications.
To re-enable the plugin you can remove the redirection-disable.txt file.
You can also disable the plugin by adding:
define( 'REDIRECTION_DISABLE', true );
To your wp-config.php file. This should be added alongside the other define statements. If you are unfamiliar with PHP it is simpler to use the redirection-disable.txt method.
Note that you may need to clear your browser cache after making this change.
My site is still broken
If your site is still broken and you suspect Redirection then you can rename the Redirection plugin directory.
wp-content/plugins/redirection
To
wp-content/plugins/redirection.disabled
Once you’ve made your changes you can rename it back.
Again, you may need to clear your browser cache.