While editing in text document the normal insert cursor changed to "OVR" (overide?)

I was tabbing back n forth in a text document to reduce spacing gaps. Somehow the "I" shaped insert cursor changed to a "black rectangle box" cursor. Now when I want to tab or insert, the black box deletes the next character, each type I advance it on the text document.

I tried to find the properties for the insert cursor(?) but could not. Pls help.

I want to have the blinking cursor returned to the "white capital I" not the "black rectangle box" cursor. I want to be able to insert the cursor and put spaces in the text when I press the space-bar, instead of having the space-bar cause the next character to delete.