Update SharePoint All List Item with PowerShell

Hi all, I need to update SharePoint list that contains 20000 rows.

My list contains 2 columns: title and URL(Multiple Line of Text) as below:

Title                              URL

Training                        http://acme/sites/school/doclib/traning.docx
Presentation                 http://acme/sites/school/doclib/presentation.docx
Movie
..
…
Row 20000

I need to use PowerShell to loop through 20000 items and update doclib in URL column to new URL like below

Title                              URL

Training                        http://acme/sites/school/newdoclib/traning.docx
Presentation                 http://acme/sites/school/newdoclib/presentation.docx
Movie
..
…
Row 20000

Can you provide simple PowerShell command to update?

Notes: Some row does not have URL, only title.

Thanks

Tee