Do not forget you will loose any modification on your current them, If you do not want to loose them you have to save them and modify them on the new version.
Steps are below:
- Back Up your current site/files
- Back up your current database
- if you can turn off all your 3 party apps / you can turned them on later one by one (they might not be compatible with version 17.x
- Download new version / unzip, upload and overwrite all the current files except config file
- login to your Admin Control Panel
- Click on upgrade database link / wait until upgrade complete
- Turn your apps ON, one by one, make sure that they are compatible with new version
Good Luck
Note: Just in case if something goes wrong, reupload your previous files that you backed up before same as database and report the error message here
