If you have previously recommended this thread, you should see a tick / check mark on the recommend button. Click the tick to undo the recommendation (the tick may appear to change to a cross as you do this.) If you added a comment with your recommendation, you will need to delete that from your facebook wall separately.
I have the facebook app on my iphone. However, I find that it doesn't update the news feed properly. Sometimes it's fine, but other times the most recent status update it will show is from several days previously - even though I've had loads of updates since then (that I can see on the news feed via the PC). Any thoughts?
Is probably the setting you have for your news feed. You can have 'most recent' or 'top stories'. The latter will show you posts at the top which have been most active or most recent commented on. The former just gives you posts in chronological order.
You should be able to set this using the news feed settings options.