Hi John,
Assuming you are not using container form, the top;co;fl;boot switch command flips back and forth between version N and N-1. Install 10.6.0.7 and return to 10.6.0.6 using 'boot switch'. To go back to 10.6.0.7 'boot switch' once more. Each time you 'boot switch' it will flip back and forth between .6 and .7.
https://www.ibm.com/docs/en/datapower-gateway/10.6.x?topic=commands-boot-switch
You will have .6 and .7 in place until your next update. If you want to preserve the N-1(10.6.0.6) version during next update, say 10.6.0.8, do the update from the N-1 side and when complete you can 'boot switch' back and forth between 10.6.0.8 and 10.6.0.6.
If you are using container form, edit the image: tag in the datapower service or, if APIC is present, edit the gateway service. You can flip back and forth between any versions simply by editing the image tag.
https://www.ibm.com/docs/en/api-connect/10.0.8_lts?topic=subsystem-overriding-default-datapower-image-version-license