Can we change an agreement once it's made? I.e. changing the value of assets

For this question, we will assume that you have not yet signed the draft separation agreement that your Ex has sent you as a “layout”.

Even though you have indicated that you agreed to everything, in law you are merely in the negotiation stage; neither of you has committed yourself to any specific form of that draft.  In fact, your proposed insertion of a new clause (stipulating no spousal support) is essentially a counter-offer in answer to her initial offer as reflected in that first draft.   In response to that, your Ex can add, delete or change the proposed terms (for example, by changing the value of assets) and present them to you for your re-consideration.   This is normal back-and-forth that is part of a successful negotiation.

After this process – and once you and your Ex are able to agree to all the terms and reach consensus – you both need to both sign the written separation agreement and have it witnessed.  Then it become binding on you both.