Hi I would like to ask if is it possible to change the default link of SharePoint link from office 365 to a specific site collection? I'm still new to SharePoint and I'm really having hard time working on this. thank you in advance. See Screenshot below
No, but you can add custom tiles to your App Launcher (if you have the correct permissions).
In the Office 365 admin center, in the left navigation choose Settings > Organization profile > Add custom tiles for your organization.