mt940.html

Statement Date<input id="statementDate" type="date"></input>
Statement Start<input id="statementStart" type="number"></input>
<button id="go" >Go!</button>

<textarea id="mt940" cols="120" rows="50">
:20:ACME2203100000001
:28C:6789/1
:60F:C220310USD0,00
:61:2203100310C2000,00NTRF00000000//00000000
0
:86:00000000 0    
:62F:C220310USD2000,00
:64:C220310USD2000,00
-
:20:ACME2203100000002
:28C:69/1
:60F:C220310USD0,00
:61:2203100310C2000,00NTRF00000000//00000000
0
</textarea>

mt940.js

function reformatFile(){
  
  let start = statementStart.value;
  let date = statementDate.value;
  const lines = mt940.value.split("\n");
  
  //Fix the start number
  if(start){
    for(let i= 0 ; i < lines.length; i++){
      if(lines[i].startsWith(':28C:')){
        lines[i] = `:28C:${(start++)}/${lines[i].split('/').pop()}`;
      }
    }
  }
  
  if(date){
    date = date.split("-").join("").substring(2);
    
    for(let i= 0 ; i < lines.length; i++){
      if(lines[i].startsWith(':20:')  || 
         lines[i].startsWith(':60F:') || 
         lines[i].startsWith(':61:')  || 
         lines[i].startsWith(':62F:') || 
         lines[i].startsWith(':64:')){
        lines[i] = lines[i].replace(/\d{6}/,date);
      }
    }  
  
  }
  
  mt940.value = lines.join("\n");
  console.log(mt940.value);
  
}


go.addEventListener("click", reformatFile);