Data Load Rule Convert character from source to number

Summary:

Hello,

I need to load data from an external source where one field contains a single character, ie, A, B etc. I need to convert this to a number in the target app, so A = 1 , B = 2

Very new to this environment, where do I define this? Many thanks...

Content (required):

source value = A

Target value = 1

Version (include the version you are using, if applicable):

22.0

Code Snippet (add any code snippets that support your topic, if applicable):