T SQL Cte Previous record calcuation

I'm using SQL Server 2016.

I have the below table:

SKU     Shop       Week    ShopPrioirty    Replen      Open_Stk  Open_Stk Calc111     100         1            1           0           17         NULL111     200         1            2           2           NULL       NULL 111     300         1            3           0           NULL       NULL111     400         1            4           0           NULL       NULL222     100         2            1           5          17          NULL 222     200         2            2           5          NULL        NULL222     300         2            3           5          NULL        NULL222     400         2            4           5          NULL        NULL

This is the desired result:

SKU     Shop      Week    ShopPrioirty    Replen      Open_Stk  Open_Stk Calc111     100         1            1           0           17         17111     200         1            2           2          NULL        17111     300         1            3           0          NULL        15111     400         1            4           0          NULL        15222     100         2            1           20           17        17222     200         2            2           15          NULL       12222     300         2            3           12          NULL        7222     400         2            4           10          NULL        2

I need to update the 'Open_Stk Calc' based on the previous row:

'Open_Stk Calc' - IIF('Replen'<=IIF('Open_Stk'>=0,'Open_Stk',0),'Replen',0)

I am using a CTE to update a row based on a calculation of the previous rows. This is my SQL:

    ;WITH CTE AS     (        SELECT        SKU,        [Shop],        [Week],        [Store_Priority],        [Replen],              [Open_Stk],        [Open_Stk Calc],        FIRST_VALUE([Open_Stk]) OVER ( PARTITION BY [SKU] ,[Week] ORDER BY [Store_Priority] ROWS UNBOUNDED PRECEDING)         - ISNULL(SUM(IIF([Replen] <= IIF([Open_Stk]>=0,[Open_Stk],0),[Replen],0))        OVER (PARTITION BY [SKU] ,[Week] ORDER BY [Store_Priority] ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), 0) AS CurrentStock    FROM [tblTEST])    UPDATE CTE     SET [Open_Stk Calc] = CurrentStock

However, this produces the following result:

SKU     Shop       Week    ShopPrioirty    Replen      Open_Stk  Open_Stk Calc111     100         1            1           0           17         17111     200         1            2           2           NULL       17 111     300         1            3           0           NULL       17111     400         1            4           0           NULL       17

And not the desired result - where have I gone wrong?