Yes! You can increase your investment; however, this will create a new investment. Because of this, you can only later cancel or amend this part of the investment. 

For example, you had a $2,000 investment finalized, and you later invested an additional $1000. You can now:

• increase your total investment to $3,500 ($2,000 finalized + $1000 new + $500),
• decrease your total investment to $2,500 ($2,000 finalized + $1000 new - $500 decrease),
• cancel your $1,000 investment (before the cancelation deadline) and be left with the original $2,000 investment.
• but you could not reduce your total investment below $2,000.