I'm on stock WP 8.1 now , will I have to reset my phone when I upgrade to Windows 10 not Insider build or I can just upgrade and use my apps without any problems. Thanks in advance
When it comes OTA, you will just upgrade and life goes on.
Even with Insider it is the same, but with earlier Insider releases the upgrade process went through many testing phases which caused Insiders to have to reset for an optimal upgrade experience. It now works well enough on current insider releases though, so the OTA upgrade should be fine.
You'll be able to upgrade without a reset and keep all your apps, however having no problems is not guaranteed, it's always best to do a reset if you want the best experience which is true of any OS including PC.