how to uninstall version 8 erpnext and how to install version 7 with all data we create?
First, you’ll need to backup all your data, databases, custom doctypes, custom apps.
To uninstall ERPNext and Frappe, you’ll need to remove your frappe-bench folder and remove the frappe user. (See link here: How do I uninstall erpnext my server?)
Afterwards, install V7 by following the steps in this thread : Install old version frappe
Setup your files, reinstall custom apps, Restore your DB.
Run Bench Update and Bench Migrate.
And that should be it.
PS. Lot of devs here will recommend that you install frappe on a fresh server/vm to avoid errors. So if there’s an error during installation, you might need to reformat the server/computer.