You can upgrade to High Sierra:
Get macOS High Sierra
Before upgrading:
1. Backup. Anything can happen at any time,, so you should always keep a good backup. Make a bootable clone SuperDuper! or CarbonCopy Cloner or use TimeMachine.
2. Run Disk Utility > First Aid from your Recovery Partition
3. If you have issues on your present system don't compound them by installing over them. Solve them first then upgrade.
4 Check HDD for free space. At least 8.8GB of available storage. Preferably much more.
5. Check your applications for compatibility here…http://roaringapps.com/apps.
6. Disconnect all peripherals, including USB hubs.
7. After downloading the installer, it will be in /Applications folder. Duplicate it and then drag the copy it to another locations like Downloads, if you want to save the installer.
8. Be patient, the download takes a long time and the install may look like it has died at times. Ignore it.
9. If you have any doubts about upgrading, install the new upgrade on an external disk and test drive it first.