Hi @GabrielaS !
It is totally possible to upgrade from the basic pricing plan of Shopify to Shopify plus as long as you can cover the fee. Shopify Plus is meant for high-volume business with the fee starts at $2000 per month - clearly not a comfortable price.
Therefore, I recommend you consider other pricing plans of Shopify such as the Advanced Shopify which starts at $299/month.
Anyway, if you still want to upgrade to Shopify Plus, you only need to contact the Shopify Plus team by filling the form and they will give you all the detailed instructions.
Best of luck,