import pandas as pd

d = {'Gender': ['F','F','F','F','M'],
    'Age': ['0-17','0-17','0-17','0-17','+55']
    }
df = pd.DataFrame(data=d)
print(df)

  Gender   Age
0      F  0-17
1      F  0-17
2      F  0-17
3      F  0-17
4      M   +55

df.loc[df['Age'] == '0-17', 'Age'] = '17'
df.loc[df['Age'] == '+55', 'Age'] = '60'

print(df)

  Gender Age
0      F  17
1      F  17
2      F  17
3      F  17
4      M  55