On the new Upstox app, from the ‘Account’ section switch to ‘Upstox for investors’ mode if you’re on Upstox Pro mode
Switch to ‘Stocks’ from the navigation bar
Scroll down to ‘Orders’ in your ‘Portfolio’
Look for your Limit order and click on it
Click on ‘Modify’
Modify the Amount/Quantity
Confirm the order and that’s it
Note:
You cannot modify the validity of the order
You cannot change the order type from Limit to Market/SIP