Here’s one way to do it.
1. Fill the 6 cm cup.
2. Pour its contents into the 10 cm cup. This leaves 4 cm yet to be filled.
3. Refill the 6 cm cup and use it to fill the 10 cm cup. This leaves 2 cm in the 6 cm cup.
4. Empty the 10 cm cup and add the 2 cm from the 6 cm cup.
5. Refill the 6 cm cup.
6. Pour its contents into the 10 cm cup.
The 10 cm cup now contains 8 cm of water.