How do I change the speed in my timelapse intervals?
There is - as far as I know - no option to do exactly what you want to achieve.



How the iPhone camera app makes a time-lapse and what it means for you:

I will quote from an apple community thread since this explanation is very good I think: 


  The iPhone's time lapse camera will condense to about 20-40 seconds regardless of how long you record your time-lapse. If you record for less than 10 minutes then the frame capture is 2fps, after 10 minutes it reduces to 1fps, and reduces further at 40 minutes, 80 minutes, and so on. 


In that process, all the images in between get lost. You end up with a 30fps video and no frames in between (which you would need for smoothly slowing it back down to real-time) are saved. Therefore, when slowing down your video, it will get choppy until you can see every single frame.

Possible workarounds:


Record a normal video, then quickly change to time-lapse mode 
If you do this, you can put the clips together later on. Depending of the length of your time-lapse (which determines how fast real-time has been sped-up) you might not notice the missing part. 
Example: Record a normal clip. Quickly change to time-lapse and start recording for more than 40 minutes. As explained above, the iPhone will take 1fp2s, meaning your transition time from switching recording modes could be up to 2 seconds and you would not notice (on iPhone 6s 4K 30fps should be 350mb/min)
Take a normal video and speed the parts up in post 
This is possible and somewhat suitable for shorter clips, but you create huge files, filling up your smartphone very quickly. 
(Suitable) Example: Take a 10 min 4K 30fps video (already about 3,5GB!) . In post adapt your speed as you wish. If you don't need the flexibility of changing something afterwards, I highly suggest you export the new file and then delete the original 10 min video.


Other than that, I haven't heard of any app that does what you want.

As a note: There are apps though, where you can adjust the frame rate of video recording exactly how you want it and not make the phone decide.