Previously I have written some code on a trial site. Unfortunately, the site became production before I had a chance to complete the test code. Now I continue to develop the code on a sandbox but it can't be deployed back to the production site since it keeps saying the code coverage on the production site is lower than 75%. How can I get this fixed and deploy the code?

Previously I have written some code on a trial site. Unfortunately, the site became production before I had a chance to complete the test code. Now I continue to develop the code on a sandbox but it can't be deployed back to the production site since it keeps saying the code coverage on the production site is lower than 75%. How can I get this fixed and deploy the code?