Increase attachment size limitation with Outlook 365
This is not a client-side setting that you can control.  This is a server-side setting, which is to say, it's something that Microsoft limits (as do most internet service providers).  Since your attachment is larger than their limit, you could upload it to oneDrive and then send a link to the file on oneDrive to your intended recipient.  You should have at least 7 GB available in your oneDrive account, possibly more, depending on which Office 365 subscription you have.