Decompiling, modifying and compiling a flash object

I have never worked with flash (SWF) before but I am in a situation where I need to modify an old flash object used internally on a site. The problem is that the flash object contains a year picker that stops with 2021 which is a problem now.

The source code is not available anymore (it's nearly 10 years old) but I have managed to decompile it using an online service and thus obtaining the source. I also found the year picker's definition, which goes like this:

        ...
        {
            label:"2020",
            data:"2020"
        }, {
            label:"2021",
            data:"2021"
        }];

So the actual change is easy enough, but how do I proceed from here?

I have no idea how to compile flash, and there is no make file etc which I guess is normally used.

How do I compile the flash object again from the modified source?

Note: For various reasons it is not an option to simply skip the flash object and building the same functionality using HTML.

Solution

Based on the suggestions from Organis, this is what I did.

I downloaded the Free Flash Decompiler and opened the SWF file. This gave me a treeview of objects and code modules to the left.
I searched for "2021" since this was the last year in the dropdown.

Found the relevant section which looked like this:

 ...
 {
   "label":"2020",
   "data":"2020"
 },{
   "label":"2021",
   "data":"2021"
 }];

I tried editing the source code, but it didn't work. Got a namespace error.

Instead I edited the P Code in the right pane. It looked like this:

pushstring "label"
pushstring "2022"
pushstring "data"
pushstring "2022"
newobject 2
...
newarray <number of years>

Note that array length had to be adjusted to the total numbers of years in the range
Furthermore I had to increase the stack size etc because the Flash object crashed when loaded. I increased all these values a little by trial-and-error.
```
body
maxstack 17
localcount 3
initscopedepth 15
maxscopedepth 17
```

Hope it helps somebody in a similar situation.