2
           
            
                
     
    
    I only have XOOPS 2.2.4 installed, but I 
think it works the same way in 2.0.x.
You need to change the column users.level from 0 to 1 for those users. I suggest testing it on one user first.
Note that this will 
not send an email to the user telling him that he's activated.
To do it with a single query, you would need to provide a WHERE-condition, such as a range or list of user IDs, or a range of registration dates.
Or if you wanted to activate 
everyone who's inactivated, you could use this query, where xxxx is your table prefix: (
backup your database first)
 UPDATE xxxx_users SET level=1 WHERE level=0;