Amending Customer Account via routine

I need to generate alternate account ID by combining existing values of CATEGORY, COMPANY CODE and ACCOUNT ID for all accounts on record. I think I should write a routine to do the automated conversion, but I do not know where to attach it. How can I achieve this? Please help.

I think I will attach the routine as a Batch job, then run as service. But still on the same, I would like to know, which is the common function that can update specified fields on the ACCOUNT application?

I want to enter the Alternate Account ID in such a way that the input will be authorised.