Please download the version of Fordson that corresponds with your version of Moodle here: https://moodle.org/plugins/theme_fordson
Once you download the proper version you install Fordson just like any other plugin by uploading it in Site Administration > Plugins > Install Plugins
Install from Github
Click on the button to “Clone or Download” https://github.com/dbnschools/moodle-theme_fordson . When downloaded to your computer, unzip it. It should create a folder named “moodle-theme_fordson-master”. Rename the folder so that it is “fordson” (without quotes). You can FTP that folder to your moodle site in /moodle/theme/ directory. Or you can create a new ZIP file of the “fordson” folder and upload and install it via the Plugin Administration in Site Administration.
Whenever upgrading from one version of Moodle to another it is best to revert back to the original default theme (More or Boost) and perform the system upgrade. Significant changes occur during Moodle upgrades which can break plugins and themes. We recommend you switch to the default theme and then perform the upgrade. Once the upgrade is complete immediately upgrade Fordson to the version that corresponds with your newly updated Moodle. Upgrade Fordson and then switch from the default theme back to Fordson.