You were on the right track. The revision number can be passed in via the SvnUpdateArgs
object:
SvnUpdateResult result;
SvnUpdateArgs args = new SvnUpdateArgs();
// If args.Revision is not set, it defaults to fetch the HEAD revision.
if (revision > 0)
{
args.Revision = revision;
}
// Perform the update
using (SvnClient client = GetClient())
{
client.Update(localPath, args, out result);
}