my app has impression more than 5K but ecpm is too low....How it increases ??

Hi,

As stated here: https://unityads.unity3d.com/help/resources/best-practices#understanding-how-ads-generate-revenue

The following types are listed in escalating order of expected earning potential:
1. Ad start (the player triggers an ad)
2. Completed view (the player watches an ad to the end)
3. Click (the player clicks the ad, leading to an external product link)
4. Install (the player clicks out to the external product page and installs the product)
Your eCPM (effective cost per 1000 impressions), represents your overall blended revenue across these streams. The key to maximizing your eCPM is designing your ads to consistently generate the highest quality impressions.

So, most likely your eCPM is low because most of the impressions are of type "Ad start" which provides the smallest earnings.

There quite a few ways of improving your Impressions, most common being the rewards for players for watching the ad. You can give player In-game currency, some small advantages, and so on if they watch the ad till the end.

Hope that helps