Hi Anamaria,
Here's how I'd do it
First there is no direct way to set a sandbox to a "project label"- but you can use the project label to get the according checkpoint number:
"si viewprojecthistory --rfilter=labeled:<your label>" should help you.
The most straight forward way now would be to use "si createsandbox" to create a new build sandbox in your target directory, but as this will break if there already is a sandbox, you'll have to do more.
First check the server's target Dir for any "project.pj" files (that indicate that there already is or was a sandbox)
If you find something you can use "si sandboxes" and check whether there is a sandbox already registered for you.
If not you can try to import this sandbox or (as I'd do it) simple erase the entire folder on the server and create a new sandbox.
Lets assume you found a sandbox that is already registered for you, you can use "si retargetsandbox" to retarget the sandbox to your checkpoint number.
Calling "si resync <server's sandbox>" will to the rest.
HTH Matthias