Hi Kenny,
I tried to do this myself. I have 2 assign statements that will create substrings and replace the values.
To do this I have 3 local variables: OriginalNumber, ReplaceFirstNumber and TempSecondary.

First assign ^

Second assign ^
TextToInteger(Concat(Replace(Substr(OriginalNumber,0,1),Substr(OriginalNumber,0,1),ReplaceFirstNumber),TempSecondary))
This will replace the first digit of original number with the new number and then concat the changed number with thre rest of original stored in tempsecondary.
I noticed that while using the logic of Sachin all the numbers changed if you make that dynamic since the logic will search for 5 and replace all instances of 5.