If those are the only possible prefixes, you can do:
UPDATE YourTable
SET amount_in_rmb = CASE WHEN amount_in_rmb LIKE 'RMB%' THEN SUBSTR(amount_in_rmb, 4)
WHEN amount_in_rmb LIKE 'RM%' THEN SUBSTR(amount_in_rmb, 3)
ELSE amount_in_rmb
END
If there are other prefixes, add more WHEN
clauses. I don't think there's an easy way to skip over all letters at the beginning with SQL.