I want to upgrade from Osclass 3.9.0 to Osclass 4.1.1

Do I replace these folders only? oc-includes or oc-admin

@ahmedzake
Instructions are pretty clear.
https://osclass-classifieds.com/installation

There are just few simple steps to upgrade your osclass to latest version:

• upload osclass ZIP file into your root directory where osclass is installed
• extract ZIP to replace old files
• go to oc-admin, you will be prompted to perform upgrade
• click on "Upgrade now" button
• once finish, everything is updated and ready to use

Make sure to backup your files and database before each upgrade! In case you got stucked on upgrade, restore your database and file system.

What to do when a white screen appears and no upgrade button avilable ?! I am upgrading from EVO 4.0 to your version, but Whute screen on upgrade page no buttons, just blank page

I do not think update from evo will work, you can try to replace all files and then manually change backoffice theme to omega
You can find it in t_preference table, hopefuly it will be only problem with backoffice theme using custom functions.
Make sure to enable and check also error log.

Btw. If you remove/rename folder of BO theme in oc-admin/themes folder, osclass will use one from backup (gui).