by Ken Task.
Mr. V's chart is excellent info, but you have a tool on your server right now with which you need to familiarize yourself. Now and during/after upgrades as a reminder.
Login as Site Admin, then to go Server Environment.
On the Environment Screen - update the component - link in upper right hand corner of that screen.
Then in the moodle version drop down list which shows your current version first, choose each moodle version higher that what you have.
The screen will refresh and show environment info.
That will show your current versions of PHP + extensions as well as your current DB version.
Be sure to scroll to the bottom of that screen each time as there is sometimes additional information that must be addressed.
Greens means server environment checks out - all is good.
Yellows means you should investigate follow recommenations for optimizing your server environment.
REDS - you MUST address before attempting upgrade to the version in the drop down list.
All of that is core ... what it doesn't check are addons - additional plugins - and theme compatiblity if you are using a third party theme for your site.
For theme suggest you force your site to use boost via config.php file by adding a line to config.php above the comments at the bottom:
$CFG->theme='boost';
That will force the site to use a stock theme that comes with every newer version. After upgrading one of the things you need to do is check notifications to see if there is a link to update/upgrade plugins.
Strongly advise getting those updates to the plugins.
The addon plugins you have to check by going to
https://moodle.org/plugins/?q=
and finding what you have currently installed to see if there exist a compatible version.
And a comment about that: if an addon doesn't have a version you need for the upgrade, my take on that ... plugin goes (un-install it) for that one plugin might be a blocker to upgrading.
And one more piece of advice ... don't get in a hurry ... be slow and methodical. It's not important how fast, but it is extremely important to be correct!
'SoS', Ken