To edit a chart title that you have inserted into a LibreOffice document:
Double-click on the chart.
A grey border appears around the chart and the menu bar now contains commands for editing the objects in the chart.
Double-click on an existing title text. A grey border appears around the text and you can now make changes. Press Enter to create a new line.
If no title text exists, choose Insert - Titles to enter the text in a dialog.
A single-click on the title allows you to move it with the mouse.
If you want to change the formatting of the main title, choose Format - Title - Main Title. This opens the Title dialogue.
Select one of the available tabs in the dialogue to make modifications.
Click OK. In your document, click outside the chart to exit chart editing mode.