You can update GSTIN in the following ways:
Firstly : To update your GST number on the Open Dashboard you need to log in to Open & Go to the settings tab and click on settings. Under the settings tab, click on 'Company' and then press on 'Tax and compliance' and then click on ‘edit details’ and update the GSTIN number.
Secondly : To update your GST number on the Open Dashboard you need to Go to the ‘Getting Started’ screen and Click on ‘Explore Open’. Then you need to select ‘Accounting & Taxes’ and click on Add GST/TAN No. Once you add it Click on ‘Submit’.