How does a decrease in a company credit rating affect its financing?

Credit worthiness decreases --> hence creditors perceive a greater credit risk (or a slowly growing default risk) --> hence this raises up the risk premium they require on financing given to the company ---> which translates into an increase in Cost of Funds (Opportunity Cost, Discount Rate or Interest Rate) however you may call it.

Availability of credit decreases with fall of credit worthiness.
Also, cost of credit increases

basically credit rating of a firm or company in a sense of a silent message to the investor or the lender who feel confidence to see the A or A+ or A++ credit company to whom he is going to invest, those companies who decrease the credit rating means they borrowed more than earlier and loss their ability to pay the debts as compare the ability before the decrease.
So those companies adversely affected by the decrease o credit rating

Of course decrease in company credit, will discount its financing,in which creditors will get negative feedback about : cash flow OR/AND financial position which will raise up the cost of fund.

there are companies which specializes in credit rating if the credit rating decreases the it is possible that the company may pay higher interest on loan or it may face difficulty in getting loans

A credit rating evaluates the credit worthiness (based on qualitative and quantitative information) of a debtor, especially a business (company) or a government.
It is an evaluation made by a credit rating agency or a lender, bank of the debtor's ability to pay back the debt and the likelihood of default.
A poor credit rating indicates that the company or government has a high risk of defaulting, hence the lenders’ or bank becomes cautious and reluctant to provide any new financing to the company or government.
Also the cost of financing for the borrower with a poor credit rating is high as compared to a borrower with a higher credit rating.

It badly affects the financing as the risk analysis is done on the basis of credit rating so when it is affected it will fall among the high risk business and financing will not be approved.

Simply put.
Any decrease in a company's credit rating, will decrease its chances of obtaining financing.