If your current visa is from a previous course at the University of Birmingham, you may be able to use it for your new course at the university.
You should be able to register on your course with your visa if it meets the following requirements:
It is a Student visa issued for the University of Birmingham - this means that when you applied for the visa you used a University of Birmingham offer letter / Visa Letter / CAS
Your visa has time left on it to cover the official start date of your course
Your visa is valid-it has not been cancelled or curtailed (cut short by the UKBA)
You are already in the UK on the visa
You have completed the course your visa was issued for and within the expected course end date shown on your offer letter / Visa Letter / CAS.
You have not broken your visa conditions, for example working more than the allowed hours, staying in the UK during an official Leave of Absence period
Extending your visa
If this visa will not cover the full length of your course you will need to apply for a new student visa before your current visa expires.
You have 2 choices:
Apply before the start of your course - as you are going to start a new course you can apply for a new student visa up to 3 months before your course start date.
Apply during your course - you must submit a valid visa application before your visa expires.
Sources of help
The ISAS visa pages provide information on how to apply for new visa including the forms you need to complete, help available and when to apply.
If these pages do not answer your immigration query, please submit an online query to ISAS.