j.py

new_data=''
with open(r"C:\Users\grape\Desktop\user.txt",'r',encoding='utf-8') as file1:
    for data in file1:
        if '小强：1本' in data:
            data=data.replace('小强：1本','小强：11本')
        new_data +=data
with open(r"C:\Users\grape\Desktop\user.txt",'w',encoding='utf-8') as file1:
    file1.write(new_data)