By using WSG, Can it update a document version?
Yes, you should be able to do that.
To update existing version just as a field:POST {server}/.../PW_WSG/Document/{docGuid}{ "instance": { "instanceId": "{docGuid}", "schemaName": "PW_WSG", "className": "Document", "properties": { "Version": "<your new version name>" } }}
To create new version of a document:POST {server}/.../PW_WSG/Document{ "instance": { "schemaName": "PW_WSG", "className": "Document", "properties": { "Name": "<existing document name>", "ParentGuid": "<existing project guid>", "Version": "<your new version name>" } }}