Can I change my thermal paste

Yes you can, use this guide on how to open it up, no need to physically remove the battery:

Lenovo Yoga 720 Akku Austausch

There will be 3 screws clamping the heat plate which is attached with the heatsink assembly and an additional two or three screws for each fan.

I recommend removing the screws for each fan and unplugging the cables for each fan before attempting to remove the heatsink or fans.

Once it's remove clean old thermal paste off the CPU and heatsink and if needed use isopropyl alcohol to clean the rest.

Then apply some good thermal paste such as Arctic Silver 5 or equivalent.