Am I allowed to get a job part way through the academic year?

Hi, so this might be a silly question but I'm wondering if I apply for student finance and declare my income, am I able to then get a job within the academic year?

So, say I start my course in September and have declared part time work income but want to take on bank work to give me a boost in funds - is that allowed? And if it is, what do I have to do re student finance?

Thanks!!