You have an Eachine E68, aka Drone2, aka dji mavic clone.
You said the drone flew backward. Do you mean.......
1. When you moved the right stick forward, drone went backward instead?
2. The drone moved backward automatically without your input on the right stick?
If answer is #2, you need to "trim" the drone.
I don't have a E68. My guess is your Forward/Backward trim buttons are the ones on the RIGHT side of the controller. One button with arrow pointing UP ( for forward trim) and one button with arrow pointing Down (for backward trim).
Check your user manual for Trimming. I could be wrong.
If my guess is right, put your drone up in air, then press the UP button until the drone stops going backward.