You have to ask for a review within 21 days of getting the written decision letter. The 21 days start from the date that you receive the letter, not the date printed on the letter. It is always a good idea to keep the letter and envelope in case there is any dispute about when you received it.
You, or someone acting on your behalf, can ask for a review verbally or in writing. If you ask for a review verbally, get the council to confirm in writing that they have received the request from you. Where possible, it is best to ask for a review in writing and keep a copy of the letter.
If you want to ask for a review after 21 days has passed, get advice immediately. It will be up to the council to decide whether to accept a late request so make sure you explain the reasons why you did not ask for the review within the 21 days.