I think you're not talking about digital signatures but rather about e-signatures, which are just a stamp. I guess you're using Fill&Sign functionality. The UI to reset your saved signature depends on the Acrobat/Reader version that you have. In Acrobat XI there is a small triangle to the right of the "Place Signature" button. Click on it and you'll get a drop-down list with choices that include "Change Signature". This is the one you want. In Acrobat DC, click on "Sign". It will show you your saved signature with a "-" to the right of it. Click on "-" and you will be able to delete your saved signature and create a new one.