Clearing the Data of a old drive

[Decan Frost]

Hello gentlemen.
I have a TrueNas Mini X.
I correctly have 3 drives out of 4 bays filled and one of them is an old 8GB that i need to replace.
The other two are 16gb's and are not relevant to this.
I've purchased two new Exos 20GB that i want to add to my pool.
So my plan is to add the first 20gb in the empty bay, clear the old 8GB from data, remove it and add the last 20gb.
I've tried searching online but couldn't find any relevant help on this.
Is this even possible?

Assume that i have no idea what i'm doing

Thanks in advance