How do I allow a user to upgrade their subscription

Hi we have created a number of memberships, what we would like to do is allow a user to upgrade their membership level

i.e.

level1 £10pa

level2 £20pa

level3 £30pa

so a user registers and selects a level 1 subscription, after x amount of time they decide they would like to upgrade to a level 3 membership is this possible do you have an example or guide I could look at

thanks