I get asked this often . It is quite straightforward.
Assuming 7 is on C (if not, use the appropriate drive letter).
1. Copy and paste bootmgr and the pale yellow Boot folder onto C.
(You will get a message bcd is in use and can't be copied - skip it and copy the rest).
2. Open an elevated cmd prompt and type:
bcdedit /export C:\Boot\bcd
then press enter. Close cmd prompt.
3. In Disk Management , rt click C and Mark as Active. Reboot.
I do this as a matter of course. I copy the bootfiles ( including bcd) as above - onto all the o/s partitions.
Then if needed, just marking any one of them Active switches system status to that partition ( only works on Primary partitions, of course).