I recommend using the Upgrade MODX extra for updating. Take a look at the instructions for which versions are safe to skip and which should be installed step by step.
Also I recommend checking your site for security breaches, as there were some for all versions below 2.6.5. That was quite some time ago, so your site might be hacked at this point. Make sure to check for that and also update more frequently in the future to keep things safe!