There are two ways to change your account's investment choices: selling your portfolio and reinvesting in a new one, or redirecting contributions into a new portfolio type.
How often can I make changes to my investment allocations?
You can only sell off a portfolio to buy a new option twice per calendar year. However, you can start contributing to a new, additional portfolio, as many times as you'd like.
Can I re-balance the asset allocation in my Embark account?
No, you cannot rebalance the asset allocation in your individual portfolio(s), but you can tweak your overall investment options up to twice a year.