How to update sharepoint list column value

I have sharepoint 2013 list with 4 columns. One of the column is Person or Group. I have hyperlink to user profile picture.
My problem is I have hyperlink to get employee picture say for example: http://mycompany.test.com/test_mypicture1.jpg

I want to replace all 300 values in this column by removing prefix "test_" in the hyperlink. How to acheive this using powershell? I would like to see the value as - "http://mycompany.test.com/mypicture1.jpg" and the next value as
"http://mycompany.test.com/mypicture2.jpg" and so on... until mypicture300.jpg.

All I want is remove test_ in the hyperlink and keep everything same in hyperlink.

Thanks