The left border as you call it is referred to as a "right indent" in Word Jargon.
That is a part of the paragraph formatting contained in a paragraph mark. It is not in the document, itself, so there is nothing to replace with Word's replace function.

You certainly could go to that point and press the Enter key.

If you were formatting your document using Styles, you could modify the style and that would work.

Is this text pasted from some other document or website?