I have been asked by a customer to send him an estimation but change it from being in Pound to instead be in Euro. But when I try to amend, it's not doable?

Hello William-wright-s, 

Welcome to the Community page,

So what you will need to do is make sure you have multi-currency turned on, you would also need the customer to be under the currency as well and then would need to delete that estimate that is in pounds and create a new one for them in euros to then send to them again.