How to delete "Sharepoint" from Share Pint Online top bar

Hello Nima-DE 

it's not support to: https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/portal-branding 

What not to do

The following list contains the key things not to do when you apply branding to your portal.

Don't:

• Override branding of the Office 365 Suite Bar.
• Customize branding for personal sites.
• Implement by default your custom branding by using custom master pages.

But if you do want to do it, maybe this will help:

https://docs.microsoft.com/en-us/answers/questions/326418/how-to-remove-the-34sharepoint34-header-name-in-th.html

My througts about the SharePoint start site:

Work with this site, it's a good overview for your users, and this are the informations, that will be appear in your SharePoint App Bar, so, let the link and train-train-train your user's.

Regards, Dave