If there is a bad use record, first of all, you should return all the overdue ones, and then maintain a good repayment habit. After personal credit is sub-healthy, it will not get better all at once. Restoring credit information is a long-term project.
However, if the sub-health of personal credit is not too serious, and it has been recorded for more than 2 years (general banks value credit information within 2 years), they can still try to apply for a mortgage, but they should be prepared to be rejected.