It's crucially important not to get the companies mixed up, so you'll want to name them differently and confirm that the backup results are in line with how much data is in the company you're copying.
Start by going to to the Copy page and selecting "New Copy". You'll now need to select a company to copy from, a company to copy to, and the "as-of" date and time. Your backed up data as-of this date and time will be copied to the company you're copying to.
You'll need to confirm the copy before it will start.
The time required for a copy to complete depends on how much data needs to be copied. The average copy takes 30-40 minutes, but a large one can take multiple hours.