In the menu, select assumptions/financing/investments. Then select the Investors tab. Select the investment round whose date you wish to change. Then change its date.

If you want to change the date of the founders' round, you will need to change the date of founding. See related question below.

Related Questions:

How do I change the founding date for my company? / How do I change the date of the founders' investment round?

What is the right timing for investment rounds?