Spontaneous modifications of this kind (during the rental) are not up to us but rather to the local car rental company. Any changes to your reservation during the rental period therefore need to be requested directly at the local rental company.
Extending the rental won’t be an issue only if the car is still available after the reserved period. Please note that the price for additional days will be charged depending on its terms. That means, the car rental company may apply a different (oftentimes higher) rate than the original one.
Returning the rental car earlier is generally not a problem given that the return will take place during opening hours. Always notify the rental company when you wish to return the car earlier. Please note that unused days will not be reimbursed by the provider.