To add or delete rows or columns; Place the mouse cursor on the beginning row or column and while holding the left mouse button, move the cursor down (rows) or across (columns) to the desired limit, let go then place the cursor on any row or column and right click to bring up the context menu to either add or delete columns or rows.
Deleting rows or columns is straight forward just pick what you want deleted and it's gone, everything that remains is either moved up (rows) or to the left (columns) and they assume the previous numbering or lettering.
When adding rows or columns the number of rows or columns you pick will be the number added, what remains is either moved down (rows) or to the right (columns) and any previously occupied rows or columns will be given new numbering or lettering;
Tip
Be careful that you do not go beyond what you want to delete when deleting, it's gone unless you "undo the last action"; But, you can highlight over an occupied row or column when adding rows or columns without the fear of losing anything.