Upgrading Cassandra from 1.1.0 to latest
what would be the procedure to update from my 1.1.0 version to the
  latest version without having any downtime ?


You're going to need to do 3 upgrades in-total:


upgrade 1.1 to 1.2
upgrade 1.2 to 2.0
upgrade 2.0 to 2.1


And you'll want to follow these steps for each upgrade:


Stop the node.
Back up your configuration files. Depending on how you install the
product, these files may be overwritten with default values during
the installation.
Install the new version of Cassandra.
Configure the new product. Using the backups you made of your
configuration files, merge any modifications you have previously
made into the new configuration files for the new version.
Configuration options change often, so be sure to double check the
version restrictions for additional steps and changes regarding
configuration.
Start the node.
If you are upgrading from a major version (for example, from 1.2 to
2.0) or a major point release (for example, from Cassandra 2.0 to 2.1), upgrade the sstables on each node.

$ nodetool upgradesstables
Check the logs for warnings, errors and exceptions. Repeat on each
node in the cluster.


For questions on those steps, refer to the DataStax Upgrade Guide.