In the case you propose, you would need to update the entire list in the value for the tag 2.
In which case you can just use the firebase blocks to do this.
(I have advised you several times about storing lists to tags, instead of using firebase as it is supposed to be used, with a tag structure for each value...)
you cannot change only one item of the list stored under the tag "2". You have to update the whole list with the new value for the first item. You can do it with web component (patch) or with firebase blocks.
But I would consider TIM's advice on modifying the data schema to make it easier to modify each element
using tags 0,1,2... will create a Firebase Array, which will not behave in the same way as when you use alpha-numeric tags (e.g. 0000, 0001,0002,0003 ...)