Removing previous cached element mapping can be done as follow:
Under the Element Maps dialog, click the Advanced button, select the element types (one at a time) that you are deploying (it could be content type definition, template definition, or others). Under “Source to target mapping,” select all mapping association and click the Delete button. Then, click Update.
Back to the Element Maps dialog, you may click on the “Guess All” button, and MSM will correctly map the source object to the target if it already exists on the target server.
This is useful if you attempt to deploy an archive using MSM, where an object of the same name already exists on the target server, and was installed using an MSM archive created from a different source server; clicking “Guess All” will add the object with the same name to the target server but with a different id.