If I understand you correctly, you want to change the Initial Value of retained tags from the PLC, right?
(Initial value of a retained tag is the value it gets when you download the program on the first time, or you call Init retained tags). In this case, the values are compiled into machine code, so it's not possible.
If you want to just change values of retained tags, then you can create HMI screens for it, and maybe also edit the values from the UniApps (I need to check if it's possible, but even if can be done, then it's very uncomfortable because it was not meant for it)