If you’re not already on 1.8.1, you’ll need to upgrade to 1.8.1 as it makes changes to the file format used for storing data on disk.
1.8.1 has different resource requirements, so it’s worth checking those out before you upgrade:
http://docs.couchbase.com/couchbase-manual-2.5/cb-install/#resource-requirements
(That doc is for 2.5 but the requirements are the same)
You’ll then need to go from 1.8.1 to a 2.x series.
From 2.x you can upgrade to 3.x.
You can see more here:
http://docs.pub.couchbase.com/couchbase-manual-2.5/cb-install/#upgrading
http://docs.couchbase.com/admin/admin/Install/supported_upgrade-paths.html
Cheers!