PrestaShop provide a One Click upgrade module that help you upgrade your stores. This is very useful tool if you store is clean installation of PrestaShop with default themes and you have not installed any 3rd party modules or theme.  But in real live store, it is amost not possile to have clean installaiton of PrestaShop and default theme.  Usually you will install your faviortite commercial theme and 3rd party modules to implement your additional requirements that default PrestaShop cannot be met.

Here is recommended stesp if you want to upgrade your store successfully without spending times to do trouble shooting.

A. Before Upgrade

1. Check Compatiblity of your theme and 3rd party modules.

You must make sure all 3rd modules and theme you are using are still compatible with your newer prestashop version that you are going to upgrade to.
It is better to contact the vendor of theme or modules you are using, to confirm the compatibility of the theme and modules.

2. Newer version of theme and 3rd party modules availability

If any of the current version of theme and 3rd party modules you are using are not compatible with new version of PrestaShop, you may need to contact your vendor to request a newer version of theme and modules that are compatible with your newer version of your PrestaShop store. And get all newer version of theme and modules ready for upgrade purpose.

3. Testing the new version of PrestaShop, theme and 3rd party modules

You must verify and test new version of PrestaShop and modules at testing environment to make sure they are working fine and meet your requirements. 

4. Back up your store

Please make a full backup of your store before applying any changes.

A. back up of all store files
B. backup of store database.

5. Upgrade 3rd party modules/Themes

We recommend to upgarde the 3rd aprty modules and theme first before upgrade you store. Usually newer version of modules and theme should be compatible with your current version of PrestaShop, otherwise, you will have to uninsall and delete all modules first before upgrade.

Because during insallation of 3rd party module you may have installed override classes/mehods that is based on current version of PrestaShop classes/methods specification. If you do not uninstall them, when the store is upgrade, those old override classes/methods will cause errors and some may lead your store to malfunction.

Note - All agile module will not remove/delete any business data during uninstall, it will only remove configuration data and related HOOKs. It means once you install the module back again, all your business data related to the module is still there.  

Note - Some 3rd party module uninstallation will remove business data related to the module, please confirm with the vendor of the modules on how to hanlde the upgrade.

B. Upgrade your store

Perform store upgrade (based on the upgrade method you choose). There are different way of upgarde and miugrate you store, please read PrestaSgop User's Guide on how to upgrade your store, choose the one best fit your needs and requirements.

C. After Upgrade

1. Delete class index cache

During store upgrade, PrestaShop will er-generate class index cahce, the new generated class index file (see below) will not include the installed override class information(It seems a bug of One Click upgrade module). So you will need manually delete this file and make it to be re-gernated automatically.

Delete following Class Index cache file so it will be re-generated automatically when you prestashop store.

YourStoreRoot/cache/class_index.php