How to modify df column

[boring_accountant]

Since the third row in your example is the 2nd row of data and that Python indices are zero-based, you can slice the dataframe using [1:] as done here:

print(df)
df.loc[1:, 'Design'] = df.loc[1:, 'Design'] + ':POJK:WAT'
print(df)

Output:
  Category Design Name  Rank
0   Group2    UIK  DUT     5
1   Group1  TYAHU  HYT     1
2   Group1   JKAL  PAT     2
3   Group3    PLK  JKL     3

  Category          Design Name  Rank
0   Group2             UIK  DUT     5
1   Group1  TYAHU:POJK:WAT  HYT     1
2   Group1   JKAL:POJK:WAT  PAT     2
3   Group3    PLK:POJK:WAT  JKL     3