If you need to change the Deleted Items Retention Policy from the default 30 days to something else, e.g. 90 days. It can be done using a simple command in Exchange Online PowerShell (see my previous posts on how to connect to MS Online).
When you are connected using PowerShell, the following command, will set the Deleted Items Retention Tag Policy to 90 days (instead of the default 30 days):
Set-RetentionPolicyTag "Deleted Items" -AgeLimitForRetention 90
See the article Retention settings in Exchange Online for further reference.