If your document expires after you have added it to your Yoti, you can replace it with a new version by going to My Yoti Tab > ID Documents > Add ID Document > Select Country and choosing the relevant document type.
If your document expires, and you do not update it, you will still be able to use your Yoti, but there may be some limitations to what you can use it for. For example, opening a new bank account requires a valid document to comply with legal checks such as KYC (Know Your Customer) and AML (Anti-Money Laundering).