Changing way:
You can do it in two way ->>
1| Find in the top bar click on My Account and select Account Settings.
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2| Click on edit beside Ad Display Preference.
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3| Now from the drop down menu list, select your desired font and click on Save Changes at the bottom.
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From now the fonts will be changed for all your ad unit ads.Method 2 : To change fonts for individual ad unit
If you want to change the font face for an individual ad unit, here is a simple tutorial.
1| First log in to your Adsense account and at the top click on Adsense Setup.
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2| If you want to make a new ad unit then click on Get Ads else click on Manage Ads.
If you have selected Get Ads :
2.1| Click on AdSense for Content, select your ad unit and then click on Continue. Please note that this update is only for Ad Unit, not for Link Unit.
2.2| Now select your ad format, color and this time you will see a new option called Fonts.
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Just select your desired fonts and at the bottom click ok.
News link.Adsense blog