First, we recommend navigating to Ads > Ad Library and taking a look at what type of campaign and ads you currently have running. You will want to expand whichever campaign has the live status attached to it. By doing that, you should get a full look at the type of campaign, audience and ads you're currently running.
For example, this customer is running a Prospecting campaign. The audience is an Attribute audience and the type of ads being used are static.
Once you have that information, navigate to the Create Ads button in the top corner. You'll then want to choose the options we saw above - so Prospecting, Attribute and then Static. You should then click the dropdown where it says Group for Ads and choose the ad group that already exists.
No need to make any changes to the information that automatically fills in - from there, simply choose next and it will take you to the place where you can upload or create your new ads.
While creating ads, if you were to choose a different type of campaign that isn't within your budget (like retargeting), a different audience than the one you're currently targeting (like lookalike), OR a different type of ad that isn't within you budget (like dynamic) - you would see an error message. That's why it's so important to know exactly which presets your current running campaign has and choose them.
Pausing Old Ads
If you want to pause your older ads, you can do so by navigating back to Ads > Ad Library > expanding your campaign and ad group & then hitting the green On/Off switch to be in the off position.