Change numbering in numbered lists

Hi Community,

I'd like (as I couldn't find any way to do this but manually) to have an option to change numbering with numbered lists.

Currently :

1. List item 1
2. List item 2
  1. List subitem 1
  2. List subitem 2
...

What I'd like (as an option settings)

1. List item 1
2. List item 2
  a. List subitem 1
  b. List subitem 2
3. List item 3
  a. List subitem 3
  b. List subitem 4
...

Thanks in advance for any hint!