how to change type validation error messages?

Unfortunately Microsoft didn't expose any interfaces to change the default messages.

But if you are desperate enough to change these non friendly messages, you can do so by creating validation attribute for decimal, creating corresponding validator and finally register it with DataAnnotationsModelValidatorProvider at the application startup. Hope this helps.

UPDATE:

Sample below

Step 1: Create validation attribute

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Method | AttributeTargets.Parameter | AttributeTargets.Property, AllowMultiple = false)]
public class ValidDecimalAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext) {
        if (value == null || value.ToString().Length == 0) {
            return ValidationResult.Success;
        }
        decimal d;
        return !decimal.TryParse(value.ToString(), out d) ? new ValidationResult(ErrorMessage) : ValidationResult.Success;
    }
}

Step 2: Create validator

public class ValidDecimalValidator : DataAnnotationsModelValidator<ValidDecimal>
{
    public ValidDecimalValidator(ModelMetadata metadata, ControllerContext context, ValidDecimal attribute)
        : base(metadata, context, attribute)
    {
        if (!attribute.IsValid(context.HttpContext.Request.Form[metadata.PropertyName]))
        {
            var propertyName = metadata.PropertyName;
            context.Controller.ViewData.ModelState[propertyName].Errors.Clear();
            context.Controller.ViewData.ModelState[propertyName].Errors.Add(attribute.ErrorMessage);
        }
    }
}

Step 3: Register the adapter in Global.asax under Application_Start() method or Main() method

DataAnnotationsModelValidatorProvider.RegisterAdapter(typeof(ValidDecimal), typeof(ValidDecimalValidator));

Step 4: Finally decorate your property in your model with this attribute

[ValidDecimal(ErrorMessage = "Only decimal numbers allowed")]
public decimal CPEHours { get; set; }

Hope it helps.