You can use "one" like: "if one wants to do this, one will do this."
You can use "his or her" like: "if the user wants to do this, he or she will do this."
Some writers like to rotate "he" and "she". So sometimes they write "if the user wants to do this, he has to do this first." Then later, in another paragraph, the writers write, "The user doesn't like this widget. She prefers that widget."
You can use "they" as a singular pronoun.
You can use "one" like: "if one wants to do this, one will do this."
You can use "his or her" like: "if the user wants to do this, he or she will do this."
Some writers like to rotate "he" and "she". So sometimes they write "if the user wants to do this, he has to do this first." Then later, in another paragraph, the writers write, "The user doesn't like this widget. She prefers that widget."