The following code snippet edits a tier price.
CatalogManager catalogManager = CatalogManager.GetManager();
ProductPrice price = catalogManager.GetProductPrices().Where(pp => pp.Id = guidOfTierPrice).Single();
price.ParentId = product.Id;
//if you want to change the parent
price.Quantity = quantity;
//if you want to change the quantity
price.Value = therPriceValueAsDecimal;
//if you want to change the tier price
First, you must get an instance of the CatalogManager. Then use the GetProductPrices method to get the prices for product. According to the changes you want to make, you must set the ParentId, Quantity or Value (changes the tier price). Finally call the catalogManager.SaveChanges method.
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