How Can I change the machine name of product type
You can alter the machine name through the database to change the product type. See table commerce_product_type and alter the 'type' column. 

Make proper adjustments in commerce_product for products using the old type. You could execute something like the following SQL query

UPDATE commerce_product SET type = REPLACE(type, "tv", "lcdtv");


Be sure to test this on a local or development version of the site.

UPDATE: As mentioned by Clive, you also have to update field data/revision tables.

You'd want to run UPDATE field_table_name SET bundle = REPLACE(bundle, "tv", "lcdtv"); on any field_data_* or field_revision_* table that relates to your product type.