Hi,
If you want to change only the Activities label in the Quick Create menu, then;
- Go to "Activity" entity in solution
- Navigate to "Messages"
- Sort by "Default Display String" in ascending
- Find the first "Activities" message in the list
- Update the "Custom Display String" field with the one you want
- Publish the entity
If you also want to change "Activities" labels in the related records, you can change "Display Name" and "Plural Name" of the "Activity" Entity.
Regards,