How do I switch my career from banking to administration/back office?

Find a new job in the sector you want to switch your job too. Gain knowledge and information about the sector. Consult any career counselor and secure your new job before leaving your current job.

If you want to switch your career do it as soon as possible as it is not that hard to do it but do it only if you know all the information about the course you want to go for and pretty confident about it as well.