Can i claim HELOC loan used for improving primary residence this year when i inadvertently said 'no' to it last year?

You can claim qualified mortgage interest this year, whether you claimed it last year or not.   As long as you can show that you used the proceeds to buy, build, or substantially improve the home, an IRS inquiry would not be an issue.   See below for more information on this requirement.  

In most cases, you can deduct your interest. How much you can deduct depends on the date of the loan, the amount of the loan, and how you use the loan proceeds.

Some other conditions that must be met (there are more here😞

• The loan or line of credit is secured (put up as collateral to protect the lender) by your main home or a second home.
• The loan or line of credit must be used to buy, build, or substantially improve your home.  
• You can only deduct the portion of the loan or line of credit you used to buy, build, or substantially improve the home that is used to secure the loan or line of credit.   If you’ve ever used part of this loan to pay for things other than this home, you cannot deduct the interest from that amount of the loan, even if the transaction didn’t take place this year.

From Pub 936 -  A mortgage secured by a qualified home may be treated as home acquisition debt, even if you don't actually use the proceeds to buy, build, or substantially improve the home. This applies in the following situations.

1. You buy your home within 90 days before or after the date you take out the mortgage. The home acquisition debt is limited to the home's cost, plus the cost of any substantial improvements within the limit described below in (2) or (3). 
2. You build or substantially improve your home and take out the mortgage before the work is completed.  The home acquisition debt is limited to the amount of the expenses incurred within 24 months before the date of the mortgage.
3. You build or substantially improve your home and take out the mortgage within 90 days after the work is completed.  The home acquisition debt is limited to the amount of the expenses incurred within the period beginning 24 months before the work is completed and ending on the date of the mortgage.   Examples

The IRS will not object if you change how the HELOC was used.  However, as pointed out, if you take out a loan on your home, improvements are only qualifying if made within 24 months of taking out the loan.  Improvements after that do not qualify as acquisition costs for the interest deduction, even if you used the loan for actual improvements. 

If you have used some draws on the HELOC for things other than the Home that secures the debt, you may have a mixed-use mortgage. Let the community know if you need help figuring that out.