Can't find where to change my signature in the new Outlook

if you'd rather not have it added on every email, skip the defaults and add it yourself. when writing an email, open the Insert tab and click Signature, pick the one you want. also, if you have more than one account in the new outlook, every account gets its own signature, so set it up for each.

Gear icon top right > Accounts > Signatures. Make the signature, then set it in the two dropdowns for new messages and replies/forwards, otherwise it saves but never gets added. Your old one from classic Outlook doesn't carry over, so you'll have to paste it in again.