Hi Dimitris,
You can rename your fiscal periods in the Classic UI if you have administrative access. Navigate to Administration. --> Finance --> Setup
Select Entities, then click on the name of you Entity and select the fiscal periods tab.
You will see a list of all your fiscal periods and the name is editable from here. Depending on the age of your system and how your fiscal policies are setup, you may have a lot of entries in this list. This table uses our legacy user interface but it does allow for in-line editing of the period name.
Update the name here and it should be reflected when you return to the modern UX.