How to enumerate a column with update?

Hello, I'm new to sql and need help:

I have the following columns:

Usuário | Ranking | Posição 
Fulano      300        1
Ciclano     400        2 
Beltrano    500        3

I wanted to know how to put this position 1 2 3 ... without being manual as I do but through a sql command, always adding 1 more to stay in order: 1 2 3 4 5 ... and has that to be according to the ranking, who has lower rankings gets the top positions

I need a update command to do this, because every week I will have to change the positions.

I tried to use DECLARE @component = 1, but I could not use this command in SQL. Gave Unrecognized statement type error. (near "DECLARE"). I'm doing it for phpmyadmin. Could someone help me how to do this?