You can open your 2012 file with 2015, but if you have entered newer transactions in 2015 you will lose them.  A possibility is to purchase a third party app that will merge the two files:

Resolution for '2012 quickbooks issue: 'upgraded to 2015 but still using 2012 quickbooks' available: Yes.

The 2012 quickbooks issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the 2012 quickbooks issue yourself by looking for a resolution described in the article. If it is a complex 2012 quickbooks issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2012 quickbooks support options.

For more information about resolving issues related to 2012 quickbooks in Quickbooks/QBO or to get support for the issue 'upgraded to 2015 but still using 2012 quickbooks', click the link below: