To upgrade LiteCart simply download the latest master release from the official website and follow the instructions inside the UPGRADE.md file. Particular knowledge in hosting your own applications is needed. For example configuring Apache2, setting up accounts and access for FTP, MySQL, files and folder permissions, etc.
When upgrading, the only changes you will lose are are hard modified core files. By hard modifications we mean the actual core files themselves and not soft modding through vQmod. Please note any third party files are not overwritten by upgrading.
The only way to prevent losing your hard modifications is to turn them into soft modifications by converting them to vQmods.
Both yes, and no. Some will be affected, some not. It all depends on what changes were made to the new version of LiteCart. This is a perfectly natural cause from upgrading. If an add-on or modification stopped working it indicates that there most probably were changes made to LiteCart to improve the original feature, and for users' best experience.
We recommend running a development platform for testing. Meaning an exact copy of your production but used for experimenting. The Add-ons Store has a great tool for transfering changes between the two platforms, see Deploy Changes.
All application errors are logged in the ~/data/errors.log file. When loading a page with errors, append ?debug to the URL and you should see extended information regarding the error.