First you'll need to get near-cli
you can install by running
npm i -g near-cli
You can view the all the state keys (for testnet for example) in your account with
near view-state <account-name.testnet>
This will store a full access key locally on your machine. Select the account you wish to clear the state of
near login
git clone https://github.com/doriancrutcher/near-clear-state.git
cd near-clear-state && npm i
npx near-clear-state clear-state --account <account-name.testnet>