The blood creatinine level of my mother aged 65 is 3.4 .she is also hypertensive but she has no symptoms of kidney problem how serious is the problem and how to proceed with the treatment

Your mother should be under the care of a Physician, a specialist. This could very well be serious, very serious.

Only a Physician can diagnose and tell you how to "proceed with the treatment."

I will add one note - my late husband was a dialysis patient with end stage kidney disease. This is nothing for you or your mother to ignore. Her kidneys could be damaged beyond repair in a very short period - if that is the problem.