1. Open developer console by pressing ` or ~ on your keyboard.
2. type "achievement_reset_all" (without the quotations)
3. Done, you're achievements in-game should have reset!

You can also reset specific achievements if you needed to.

Open developer console by pressing ` or ~ on your keyboard.
Example: type achievement_reset BMS_Example

All of the achievement commands (note: replace the BMS_Example with any of the commands shown below.)

1. Brownian Motion ; BMS_BROWN_MOTION
2. Caffeine Extraction ; BMS_CAFFEINE_FREAK
3. Calculated Trajectory ; BMS_CALCULATED_TRAJECTORY
4. Centripetal Attraction ; BMS_CENTRIPETAL_ATTRACTION
5. Convection Refinement ; BMS_FLAVOR_TRANSFORMATION
6. Dead Reckoning ; BMS_DEAD_RECKONING
7. Digital Wizardry ; BMS_DIGITAL_WIZARDRY
8. Ethically Questionable ; BMS_ETHICALLY_QUESTIONABLE
9. Futile Resistance ; BMS_LITTLE_FRIEND
10. Gray Matter Propulsion ; BMS_GRAY_MATTER_PROPULSION
11. Hyper Saturation Conundrum ; BMS_HYPER_SATURATION_CONUNDRUM
12. Inflammatory Actions ; BMS_KILL_IT_WITH_FIRE
13. Kinetic Repulsion ; BMS_KINETIC_REPULSION
14. Mega Hertz ; BMS_MEGA_HERTZ
15. Nuclear Fishin' ; BMS_NUCLEAR_FISHIN
16. Permeable Infrastructure ; BMS_PERMEABLE_INFRASTRUCTURE
17. Premature Expulsion ; BMS_CHARGING_MY_LASER
18. Prophylactic Suggested ; BMS_PROPHYLACTIC_SUGGESTED
19. Proverbial Tinkerer ; BMS_PRESS_THEIR_BUTTONS
20. Quantum Capacitance ; BMS_QUANTUM_CAPACITANCE
21. Rare Specimen ; BMS_RARE_SPECIMEN
22. Remote Detonation ; BMS_INDIGESTION
23. Resonance Procrastinator ; BMS_RESONANCE_PROCRASTINATOR
24. Unified Coupling Theorum ; BMS_COUPLING_THEORUM
25. Universal Gravitation ; BMS_FRIENDS_LIKE_THESE
26. Prolific Popper ; BMS_PROFILIC_POPPER
27. PhD in Pacifism ; BMS_PHD_PACIFISM
28. The Rarest Specimen ; BMS_RAREREST_SPECIMEN
29. One Step Ahead ; BMS_ONE_STEP_AHEAD
30. Stealth Technology ; BMS_STEALTH_TECH
31. Xen When ; BMS_XEN_WEN
32. Our Brains And Your Brawn ; BMS_BRAIN_BRAWN
33. Less Than Desirable Consequences ; BMS_LESS_THAN_DES_CONSEQUENCES
34. Orbital Trajectory ; BMS_ORBITAL_TRAJ
35. Following the Rules ; BMS_FOLLOWING_RULES
36. Doctor’s Signature ; BMS_DOCTORS_SIGN
37. Gonad Glider ; BMS_GLIDER
38. Post-Mortem Interval ; BMS_POST_MORTEM
39. The Plan ; BMS_THE_PLAN
40. Unquestionably Ethical ; BMS_UNQUES_ETHICAL
41. Contest Propellant ; BMS_CONTEST_PROPELLENT
42. Laser Immunization ; BMS_LASER_IMMUNIZATION
43. Black Mesa Decathlon Winner ; BMS_DECATHLON_WIN
44. Anti-Matter Over Mind ; BMS_ANTI_MATTER_OVER_MIND
45. Specimen GG ; BMS_SPECIMEN_GG
46. Late for Work ; BMS_LATE_FOR_WORK
47. Limitless Potential ; BMS_LIMITLES_POTENTIAL
48. Rather an Anticlimax ; BMS_ANTICLIMAX
49. Pepperoni Precipitation ; BMS_PRECIPITATION
50. Well-Done ; BMS_WELL_DONE