Is it permissible to change employees' data to avoid deductions from the year-end bonus, and what is the ruling on the financial increase obtained as a result?

If the manager is authorized to distribute the bonus according to specific rules and conditions, he must adhere to them. If he acts unjustly or oppresses, you are permitted to try to prevent the injustice or mitigate it. However, if the manager is implementing agreed-upon conditions, it is not permissible to tamper with data, and the money that was taken unlawfully must be returned to the manager, or spent on deserving employees if one fears harm from returning it.