If you are not able to set up Longview yourself, this may be an indication that you need to get someone to help administer the server.
Anyway, downgrading your Linode is simple and can be done through the Linode manager. You may need to resize your disk; search the
Linode docs for advice on downgrading/resizing.
All of your data will be moved to a different, smaller Linode within the same datacenter and you will keep the same IP address. Note that your site will be down for however long it takes. You will not need to re-install anything, but you may need to tweak the configuration of some applications (database, web server) so they work properly in the smaller memory available.
If you find that a smaller Linode does not perform well, you can always upgrade back to a larger one (with corresponding downtime).