Please follow the steps below:
Go to menu 3.02 Member Types -> Select the previous member type -> Make changes to the Member Type -> Click on Save
Click on Edit link under Action caption Member record see here -> Scroll down to see Member Types caption -> Click on View button see here -> Select the newly edited Member Type -> Select the checkbox which says "Overwrite existing Member Type?" -> Click on Save.
Please see the video on how to extend the Member type by 1 Year
Another way you can achieve this is as follows:
Click on View -> Select the current membership -> Select the date/year -> Click Save ->It will create one additional member type and you can pay off by the administrator payment.