The preset short replies or responses to reject an incoming call with an SMS message can be edited at any time in the phone app's settings to save personally tailored replies on the OnePlus smartphone.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Phone app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  Quick responses
Step 5:  Choose a  quick response
Step 6:  Enter a  quick response
Step 7:  Tap on  Save
  1. Open the Phone app
  2. Open the menu
  3. Open the Settings
  4. Tap on Quick responses
  5. Choose a quick response
  6. Enter a quick response
  7. Tap on Save

OnePlus Instructions