Short answer

You can modify stock SIPs by following these steps:

Arrow app

Tap on Orders.

Tap on SIP.

Tap on the SIP that is to be modified.

Tap on Edit and make the required changes.

Swipe the Save button.

Arrow web

Click on Order and then on SIP.

Click on Options.

Click on Edit and make the required changes.

Click on Save.

If you pause a SIP, your orders will not go through from the next scheduled date unless you resume it.