Here’s a start: https://support.google.com/docs/answ…DAndroid&hl=en
For your specific question, if you’re on your phone:
1. Tap the row number that you want to start deleting at, so that the entire row is highlighted.
2. Tap the menu button at the upper right, then “Select cell or range.”
3. Adjust the range to reflect the rows you want to delete (e.g., 50:1000 means delete rows 50 through 1000).
4. Long-press that highlighted range on the spreadsheet, then tap the menu button in the edit bar that appears, then Delete.