I used following upgrade_code command:

/home/jishnavt/Desktop/"version 18"/odoo18/odoo_18_venv/bin/python3.12 /home/jishnavt/Desktop/"version 18"/odoo18/odoo-bin upgrade_code --from 17.0 --to 18.0 --addons-path /home/jishnavt/Desktop/"version 18"/odoo18/custom_modules/custom_sales

Detailed description :

/home/jishnavt/Desktop/"version 18"/odoo18/odoo_18_venv/bin/python3.12

Path to Python*: This specifies the Python executable that will be used to run the command. The path points to Python 3.12 in a virtual environment (odoo_18_venv), which is located on your desktop in the "version 18" directory.

Virtual Environment*: The use of a virtual environment (odoo_18_venv) ensures that dependencies specific to Odoo 18 are isolated and do not interfere with other system-wide Python installations.

/home/jishnavt/Desktop/"version 18"/odoo18/odoo-bin

Odoo Script*: This is the Odoo script (odoo-bin) that contains the core functionality for running various Odoo operations, such as starting the server, upgrading modules, and performing maintenance tasks. It is located in the odoo18 directory on your desktop.

upgrade_code

Upgrade Command*: This is a custom or additional command being run in your Odoo setup. This command tells Odoo to upgrade the code. 

--from 17.0 --to 18.0

Version Upgrade*: These flags specify the version of Odoo you are upgrading from and to:

               - --from 17.0: This indicates that the starting version is Odoo 17.

               - --to 18.0: This indicates that you want to upgrade to Odoo 18

--addons-path /home/jishnavt/Desktop/"version 18"/odoo18/custom_modules/custom_sales

Addons Path*: This specifies the path to the custom Odoo modules that you want to upgrade or include in the process. The given path points to the custom_sales module located on your desktop.

Custom Modules*: In Odoo, custom modules (like custom_sales in this case) are usually stored in separate directories and must be specified if they need to be upgraded or included in the upgrade process.

- The command may help in handling specific module-level upgrades, ensuring that data models, views, and other code aspects are correctly migrated from Odoo 17 to Odoo 18.

Regards,
Jishna
Accurates