'How to base on condition to update cell value?

Everyone, I'm absolute beginner of Python and currently leaning by myself. I have meet a problem, I hope I find talented person to teach me how to fix the issue.

I have two different excel to compare...

                               Data1.xlsx                                       Data2.xlsx
 |  Name   |   City    |  Reg Date  | Gender | Check In | Check In Date |  |   Name  |  Reg Date  |
 |Alex     | Hong Kong | 2021-06-30 | Male   |    Y     |  2021-06-30   |  |Annie    | 2021-07-01 |    
 |Annie    | Hong Kong | 2021-07-01 | Female |          |               |  |Billy    | 2021-07-02 |    
 |Bob      | Taipei    | 2021-06-28 | Male   |    Y     |  2021-06-28   |  |Cathrine | 2021-07-03 |    
 |Lucy     | Tokyo     | 2021-06-28 | Female |    Y     |  2021-06-28   |  |David    | 2021-07-04 |    
 |David    | London    | 2021-07-04 | Male   |          |               |  |Eric     | 2021-07-04 |    
 |Kate     | New York  | 2021-07-03 | Female |          |               |
 |Cathrine | London    | 2021-07-03 | Female |          |               |
 |Rose     | Hong Kong | 2021-07-04 | Female |          |               |

Use Name & Reg Date for key to merge

import openpyxl as xl
import pandas as pd
import numpy as np

dt1 = pd.read_excel('Data1.xlsx')
dt2 = pd.read_excel('Data2.xlsx')
df_merge = pd.merge(dt1, dt2[['Name', 'Reg Date']], on=['Name', 'Reg Date'], how='left', indicator=True)

   Name         City     Reg Date   Gender  Check In  Check In Date    _merge  
0  Alex      Hong Kong  2021-06-30  Male       Y       2021-06-30    left_only     
1  Annie     Hong Kong  2021-07-01  Female    NaN          NaN            both 
2  Bob       Taipei     2021-06-28  Male       Y       2021-06-28    left_only    
3  Lucy      Tokyo      2021-06-28  Female     Y       2021-06-28    left_only
4  David     London     2021-07-04  Male      NaN          NaN            both 
5  Kate      New York   2021-07-03  Female    NaN          NaN       left_only
6  Cathrine  London     2021-07-03  Female    NaN          NaN            both
7  Rose      Hong Kong  2021-07-04  Female    NaN          NaN       left_only

How to check equal both to fill Check In is Y & copy Reg Date to Check In Date ?
```
for a in df_merge.iloc[:, [7]].values:
    if a == 'both':
```

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'How to base on condition to update cell value?

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