MYSQL update query to remove spaces

You would use TRIM and update.

Just using this should do it.

UPDATE accountTable
SET ACCOUNTNUMBER = RTrim(ACCOUNTNUMBER)

If you need to RTRIM() all the accounts of a particular customer, you can use a JOIN with your UPDATE statement as follows:

UPDATE
    accounts_table
INNER JOIN
    customers_table ON (accounts_table.user_id = customers_table.user_id)
SET 
    accountnumber = RTRIM(accountnumber)
WHERE
    customers_table.customer_id = 'customer id';

If you do not have many records in accounts_table, and you want to make sure that all the accountnumber values are trimmed, you can simply apply the trim to all the records as follows:

UPDATE
    accounts_table
SET 
    accountnumber = TRIM(accountnumber);