My imei is not the same as the one under the battery

I bought an used samsung Galaxy S4 phone and after a while I found out that my imei has been changed I mean its not the same as the one under the battery, till then I thought its not a big deal but after a while I met some problems one of them is I cant connect to 3G network and my 2nd problem I found yesterday when I tried to update the android software I got that message"your device has been modified"
Now guys I want to know why am I having these problems? Can they be solved?