In Visual Studio to edit multiple lines which are vertically aligned at once press Alt and select the aligned block and then just start typing as in example 1 below. This approach overwrites the selected block.
If you want to select multiple lines / edit multiple lines in non aligned blocks you need to press Ctrl+Alt and explicitly click where you want each cursor to be and then just start typing as in example 2 below. With this approach you only add new code not overwrite existing code.
Don’t forget about the Delete button if you want to quickly remove snippets/blocks rather than add/amend them.