The first step a startup can take to improve its credit score is to start paying off its debts. This means that if you have any unpaid bills, you need to pay them off as soon as possible. The longer you delay payment, the more interest builds up over time, which negatively affects your credit score.
The second step is to create a budget and stick to it. Your budget should include income from all sources and expenses from all categories (excluding debt payments), including rent or mortgage payments, utilities and groceries. Once you've created this budget and started following it religiously, your credit score will increase because you'll be able to demonstrate good financial management skills.