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:

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 - "" and the next value as
"" and so on... until mypicture300.jpg.

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

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Rainer JeschorCommented:
question: the person/group column is a different one than the column containing the hyperlink?
Is the column with the hyperlink to the employee picture of type "Hyperlink"?


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