UPDATE statement in SQL

i have these  columns as an example:
ID  Line  Mod2, Mod3, Mod4
1    1                     59
1    2                     59    

Here is the result from the SQL script below:                  
                             
ID      LINE #      MOD1      MOD2      MOD3      MOD4
A1      1                              59              59      
A1      2                              59              59      

Line 1:  I need to move the value in Mod3 to Mod2 (if Mod2 is blank) and remove the value in Mod3 - only if the move has happened

same if there is a value in Mod4   (if Mod2 and Mod3 are blank), then move Mod4 to Mod2 and remove value from Mod4

Here is SQL script

SELECT ID, LINE,
CASE WHEN MOD2 = '' THEN COALESCE(MOD3, MOD4) ELSE MOD2 END  as MOD2,
CASE WHEN MOD3 = '' THEN MOD4 ELSE MOD3 END as MOD3 ,
MOD4 = ''
FROM

I actually need to change this to an update statement ...created the SQL select to view the results

If anyone can help out, please let me know.  Thank you