How to replace underscore(_) with space (' ') in angular view?

You can use a pipe

https://angular.io/guide/pipes

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({name: 'replaceUnderscore'})
export class ReplaceUnderscorePipe implements PipeTransform {
  transform(value: string): string {
    return value? value.replace(/_/g, " ") : value;
  }
}

Then use it like

{{ header|replaceUnderscore}}

You could also make a more generic version that takes the pattern to replace and the replacement as parameters, like @Ash-b's answer for angularJs

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({name: 'replace'})
export class ReplacePipe implements PipeTransform {
  transform(value: string, strToReplace: string, replacementStr: string): string {

    if(!value || ! strToReplace || ! replacementStr)
    {
      return value;
    }

 return value.replace(new RegExp(strToReplace, 'g'), replacementStr);
  }
}

And use it like

{{ header| replace : '_' : ' ' }} 

Here is a demo on stackblitz

If there is only one instance you can use this,

 {{ header.replace('_', ' ') }} 

or else you have to use filter

App.filter('strReplace', function () {
 return function (input, from, to) {
 input = input || '';
 from = from || '';
 to = to || '';
 return input.replace(new RegExp(from, 'g'), to);
 };
});

and use it like

 {{ header | strReplace:'_':' ' }}

hope this helps :-)