How do I add values to a new custom field on existing tasks?

[Alex Pape]

I have created a new custom field, which is multi select.

When I create a new task, I can see the field and I can give it a value. This then shows (read-only) on the task after it's saved and reopened.

I have two problems - 

1. Why can't I update/change the value in the view/edit version of my screen.
2. Why can't I add a value for this field on existing tasks, that were created before this field existed.

I am an admin on our JIRA platform and have gone through so many options and settings pages, as well as loads of previously asked questions/Google, but can't find the answer to what I believe should be a simple function.

Please help whilst I still have some hair left.

Thanks

[Nic Brough -Adaptavist-]

It sounds like the field is not present on your edit screen for that project/issuetype.  Drill down through the project admin stuff, starting with the "issue type screen scheme"