It's good practice to review your Staff Handbook and your company policies once a year.
You should also be aware of any changes to employment law and whether they could affect your policies and procedures.
If you're making changes to contractual policies, you'll need to consult with staff and get their agreement first. If you don't, you could be liable for claims of breach of contract.
If you're making changes to non-contractual policies, you can do this without getting consent from your employees.