If you have an extremely out of date site (pre 5.3.2) it is actually possible to update it to the current version of concrete5. This procedure is not guaranteed for all cases, but it has worked in my experience.
Step One: backup the current concrete/ directory and database. Run through the site and make a note of any customizations and add-ons.
Two: create an updates/ folder with proper permissions (777 or
chmod -R a=rwX). If you are using some web-hosting interface like cPanel, this means "as permissive as possible". Also, if you do not know what an updates/ folder is, this folder should be in public_html/ in your webspace, or to put it another way, it should be in the same folder as "packages/" but it will be at the end of the list, alphabetically.
Three: upload the following zip files into the updates/ folder.
- 5.5.x (Most add-ons are up to date, but be sure to check that any add-ons that your site depends on are still supported)
- ...and onwards
Four: put the site into maintenance mode.
Five: Starting with the earliest version you uploaded, replace the [site root]/concrete/ folder with the version of core in your updates/concrete5.n.n/ and visit
http://site/index.php/tools/required/upgrade/ from your browser. Repeat this until the site is at 22.214.171.124, then you can use the dashboard's update page to put the site at 126.96.36.199. After that, just keep going back to the dashboard and upgrading until you are all the way up to date.
If you are experiencing errors when you try to edit blocks, refer to this how-to about how to correct the database after going from 5.3 to 5.4: DB Fix