You should be able to edit anything in the title block or anywhere else on the page.
A template is just a LayOut file that gives you a starting point for your project. It could be entirely blank pages or it could have content like title blocks, page numbers, and other content that would be common to many projects so you don’t have to add that stuff all the time.
You could open a new project with the template, make the required changes to the title block and then copy and paste the content from each page in the previous project but it seems like it would just be easier to edit the current one.
If your page number is being done with Auto Text you may need to edit the Auto Text entry in Document Setup. Or maybe you just need to edit the text box on the page. It would be easier to help you if you were to share the LO file or at least your template so we can see what you are working with.
This is a page number starting with A. in one of the default templates. The number is Auto Text but the prefix is simple text. You can edit it to be whatever you want the prefix to be.
You can even use some symbol if you want.