Archive your email to an 'Online Archive' folder
Microsoft 365 accounts have a secondary mailbox known as an 'Online Archive' mailbox.
Moving messages into the 'Online Archive' mailbox can free up space in your main inbox.
For details of how to locate your 'Online Archive', please see the Microsoft support page titled - 'Manage email storage with In-Place/Online Archive mailboxes'.
Manage email storage with In-Place/Online Archive mailboxes
Setting retention and archive policies on emails
An alternative to manually moving emails to an archive, is to set up retention and archive policies on folders or your entire inbox. These policies can allow you to automatically archive emails after a certain number of years.
For more details about setting retention and archive policies, please see the Microsoft support page titled - 'Retention and archive policies in Outlook Web App'.