The ability to merge two company data files into one company is currently not available in QuickBooks Online. Each company is created as a separate file and can't be merged, but you can manually input the data into the company you want to keep.
In most QuickBooks Online editions, you can export a list from one QuickBooks Online company and import it into another. Find the list you need to export, select the Excel button, save it to your computer, then import it into another company.
If you want to convert or merge a company from another version of QuickBooks into QuickBooks Online, you can do so by importing a file, but this can only be done into a new company as long as it is within the first 60 days of being created. If the company is more than 60 days, you will need to set up a new one.