Change first occurrence of character x in a string - how?

[Grant Edwards]

On 2021-02-14, Mr Flibble <flibble at i42.REMOVETHISBIT.co.uk> wrote:
> On 14/02/2021 21:14, Chris Green wrote:
>> What's the easiest way to change the first occurrence of a specified
>> character in a string?
>
> By using a grown up (i.e. non-toy) programming language.

<plonk>

[gotta love slrn's scoring feature...]