Note: These screenshots are from a Mac but the process is exactly the same for Windows.
Whenever Mojang releases a new version of Minecraft, your game will be automatically updated to that latest version. Servers, however, always take longer to update. When you go to log in when a new update has come out, you will not be able to log in and will likely see a message like this:
Fear not, downgrading is easy as pie. Here's how you do it:
Step 1: Open your Minecraft launcher
Step 2: Click "Edit Profile" in the bottom left
Step 3: The following window should pop up. Click "Use Latest Version" as pointed out by the arrow.
Step 4: Now select the version the server is currently on (1.7.10)