Can you please elaborate the issue in more detail as to what are you trying to do?
If you're unable to choose your preferred OS at the time of computer startup, you can go to Windows 7, type MSCONFIG in the Run command box, press Enter, get to the Boot tab, and configure the Timeout duration and default operating system from there.
If you're stuck at reboot loop, you can get to Windows 7, use File Explorer to locate the drive Windows 10 installed on, get to the Windows > SoftwareDistribution folder and remove all its contents.
Once done, try restarting the computer to Windows 10 and see if the issue is resolved.
Hope this helps.