On the horizontal ruler you can see the tab stops for the current paragraph. If you want to change the tab stops, you should first consider the scope to which you want to change tab stops as follows:
Change the default tab stops for all documents: Use the menu LibreOffice - PreferencesTools - Options - LibreOffice Writer - General.
Change the tab stops for all paragraphs using the current Paragraph Style: Right-click the paragraph to open the context menu, choose Edit Paragraph Style, click Tabs.
Change the tab stops for one or more paragraphs: Select the paragraphs, then click inside the ruler.
In the following, you find instructions for all above mentioned tasks.
You can set a tab stop by clicking on the ruler or by selecting Format - Paragraph - Tabs. Both methods affect the current paragraph or all selected paragraphs.
Click the ruler once to set a left-justified tab. Right-click a tab icon on the ruler to see the context menu in which you can change the tab type.
To set several decimal tabs one after the other, keep clicking the icon to the left of the ruler until the desired tab type is shown, then click on the ruler.
Setting left tabs
Setting right tabs
Setting decimal tabs
Setting centered tabs
Double-click the ruler to open the Paragraph dialog.
Double-click the white area of the ruler to set one tab. The Paragraph dialog appears with the Tabs tab page open.
Move individual tab stops on the ruler using the mouse.
To move several tab stops on the ruler, press the Shift key before you click a tab. Drag one tab while continuing to press Shift to move that tab as well as all the tabs to the right of it. The spacing between those tabs remains the same.
Press CommandCtrl when you drag a tab on the ruler to move that tab and all the tabs to the right of it. This results in the spacing between those tabs changing proportionally to their distance from the margin.
To change tab type, click the tab you want to change on the ruler, then right-click to open the context menu.
To delete a tab, hold down the mouse button while you drag the tab outside the ruler.
If you want to change the settings of your default tab stops, you will find further information under LibreOffice Writer - GeneralLibreOffice Calc - GeneralLibreOffice Draw - GeneralLibreOffice Impress - General(module name) - General in the Options dialog box.
The context menu of the ruler allows you to change the displayed units of measurement. These changes are only valid until you exit LibreOffice, and they only apply to the ruler on whose context menu you made the change. If you want to change the ruler measurement units permanently, choose LibreOffice - PreferencesTools - Options - [Document type] - View and change the measurement unit there.