ok this is rather easy to do since you are already dual booting.
back up any important data from the win 7 partition before doing this as it will delete everything from there and you CANNOT GET IT BACK!!!
1 while booted into win 10, open the run command and type "msconfig"
2 select the "boot" tab and note there are 2 entries. one should be labeled c: and the other d: this changes depending on which is booted up so not important. what is important is the one that says "current OS" after it. that is the boot entry for your win 10 install. click the other one and delete it with the delete button below the list. this will remove win 7 from the boot list and it will only boot into win 10 now.
3 now go to disk management and delete the win 7 partition. all that space is now sitting there empty waiting to be added back to the win 10 partition.
DO NOT DELETE ANY OTHER PARTITION OTHER THAN THE WIN 7 ONE!!!!
4 finally from disk management right click the win 10 partition and select "extend partition" it will note the free space available and ask how much you wish to add to the win 10 partition. tell it to add all the free space and complete the process. congrats you have now removed win 7 and added all that space to win 10 to enjoy
****** it is possible (likely even) that the "extend partition" option is greyed out. not a big deal it just means that windows itself won't do it. you need a 3rd party program to complete step 4. i use
http://www.partitionwizard.com to do this. has not failed me yet. process is the same as before just in a different program. it ignores windows protests and merges the space anyway.
follow the steps and you should have no issues with the process.