Please refer to the below mentioned article/video to change the Prefix:
Article - How to add prefixes for current financial year 26 - 27 on web?
Video - https://www.youtube.com/watch?v=rih1sNlnmCA
Note: You can start using the new prefix that you have added.
If you want the new prefix to apply from April 1, please edit those invoices and select the appropriate prefix and number accordingly.