Customizing the bell's content

Does somebody knows how to customize the content of the “bell” on the taskbar ?
See the attachment to see precisely of what I’m talking about.
Thank you for your help.

What customization are we speaking of? Color? Size? Shape?

Could you please provide more information regarding this.

Content. I’d like to have next calls to do, or next tasks or meetings, or all of them. But to do .
Currently, it shows old calls, marked as planned, when they’re done since days ago. I don’t understand why it displays done calls as to do calls… And for me, it’s uselless as I work alone and I’m the only user of the CRM.