I do not use shopify . So do not know exactly . But if you read the help section of shopify, you will be able to find how to provide access to freelancer or developer or staff member etc.
Choose whichever permission level provides sufficient access for the job and then provide access to the freelancer .
You should provide only the required access for the task the freelancer is supposed to do.
In my opinion it is unwise to provide your complete account /password to a freelancer, unless you know( and fully trust) them personally.
Even if you 100% trust the freelancer...
Even if YOU ARE DOING THE WORK YOURSELF... you really would be better off if you set up a separate development server with development/test Shopify credentials. Shopify provides developer credentials specifically for this purpose... so you set things and and test without hitting your real account.