What's the best way to upgrade from umbraco 7.6 to 7.15.1 (including db upgrade)

I have done 2 umbraco upgrades recently; one is from 7.5.7 to 7.13.1 and the recent one is from 7.13.1 to 7.15.1.

During my upgrade; I have seen this problem and fix in this issue can help you for your problem(and I didn't see this problem again after doing the upgrade again, but this time checking all the auto changing files and accepting them one at a time-see details below for this) but coming back to your question; "What's the best way to upgrade from umbraco 7.6 to 7.15.1(including db upgrade)"; here are the steps that you should follow;

1. Create a backup for your project and your umbraco db before you start. If you are using Git, then things will be super easy for this.
2. Open up Nuget Package Manager for your Umbraco project and do the package upgrade using the Nuget Package Manger window or the consol. Search for UmbracoCms version 7.15.1 for your case.
3. Once you start doing the upgrade, you will see some popup windows that will ask you to approve some auto file changes(including some config files changes). As you don't want to lose some of your pre-upgrade settings, don't accept them all or discard them all, check all of them one by one, and as a general rule; if you don't have any custom changes for those files, then simply approve the change, otherwise, check your changes and make sure you don't loose anything and discard some of these file changes as a result.
4. Once you're done with your UmbracoCms upgrade(which will automatically do some dependency package upgrades), build your project, make sure all is looking good then go to your local project's umbraco back-office url, this will trigger the rest of the umbraco upgrade process and simply complete the upgrade steps by following the screens- at this point your umbraco db changes will be done automatically and it is possible that you might have some issues with some old corrupt cached files, if this happens, then simply delete App_Data/TEMP files and App_Data umbraco.config file and try again. If you see some other problems during the installation, check the logs(browser developer tools can be handy to understand the problems in this case), and fix them one at a time. It is possible that you don'T need some of your old web.config settings and they might cause some issues, simply comment out those lines and see if this will fix some of the issues.
5. Once you are done with you local upgrade, deploy your code to your testing environment, and go to the umbraco url of your test environment and follow the screens to complete the installation for your testing environment. If you see any problems, please check my notes for step 4 above.
6. Do your umbraco upgrade for other testing environments(QA, UAT, Training etc) and complete your umbraco upgrade tests. Once the tests are done, then you are ready to go live. After the live deployment, you will have to complete the umbraco upgrade one last time, but this time for the live system.
7. Always get your back-ups for each environment before you do the upgrade, so you will be ready to rollback your changes if things go wrong(which might happen as you're doing a big umbraco upgrade).
8. Final note; there are some good articles for this, please take a look to understand the process better. Good luck!