If you've got an ongoing monthly subscription you can upgrade to other plans and/or instruments right from the app (desktop only, read here how to upgrade an iOS subscription).
It is not possible to downgrade an ongoing yearly subscription, but the "all instruments" upgrade can be purchased.
In order to do that please click the 'upgrade' button below your Yousician level and profile picture in the top left corner of any tab:
Alternative way is to switch to another instrument and follow the standard upgrade procedure - you will see all options available for your profile.
The remainder of your balance will be calculated towards the new plan.
Read here if you already have a yearly subscription and would like to upgrade to the "All Instruments" plan.
Please also check the article on how to go Premium.