There are lot of articles similar with that. Ok. This is a only just my recent experience
I have VM with only one AVHD (snapshot/checkpoint) . If your VM has a lot of AVHD files , you need to locate the latest.
For example, you have 3 snapshots for VM (VHD <> AVHD1<> AVHD2 <> AVHD3) .
VHD is the parent disk for AVHD1, AVHD1 is the parent for AVHD2 and AVHD2 is the parent for AVHD3.
Merging process: AVHD3 to AVHD2, AVHD2 to AVHD1 , AVHD1 to VHD
In my case I have only AVHD1 and parent VHD (on the picture below)
View (for 2012/2012R2, folder properties 2008/2008R2) –> activate “File Name Extensions” and change AVHD extension to VHD
Go to Hyper-V Manager , click on Edit Disk on the right pane , Browse and locate your checkpoint (.vhd)
Click on the Next and choose Merge
Merge the changes to the parent virtual disk
Merging AVHD to VHD is completed.Now you can use this merged VHD disk during VM creation.