Question

 sb.Append("UPDATE Users AS u ");
    sb.Append(" SET u.Reputation = (u.Reputation + @Repuation)");
    sb.Append(" INNER JOIN Comments AS c ON c.UsersID=u.UsersID");
    sb.Append(" WHERE c.CommentsID=@CommentsID");

It tells me that I have an incorrect syntax near the syntax ‘AS’

Answer 1

You SQL is incorrect assuming you are using MS SQL Server

It should be

UPDATE u
SET u.Reputation = (u.Reputation + @Reputation)
FROM Users u
INNER JOIN Comments c ON c.UsersID = u.UsersID
WHERE c.CommentsID = @CommentsID