Oh, there are several of those. I have not tried any of them myself, but the probably best candidates seem to be:
EagleEye is probably the best choice here. I use its companion app PhotoMap, and am quite happy with it (it's based on the very same code, but rather a "lite" version lacking e.g. the feature to show pics without GeoTags and add the latter). Support is superb, issues I had were usually fixed within a week (if not faster).
For your task, it shows you the picture along with a map, and lets you visually move the "pointer" to the coordinates you want to write to the photo's Exif data:

Edit GeoTags with EagleEye (source: Google Play; click image for large variant)
Of course you can edit existing GeoTags the very same way: simply drag the baloon to where it belongs. The app has a ton of other useful features, including
- showing your photos by country or place
- include a "GPS path" to show the order they were taken (map view)
- show by year/month, distance, name, folder
And more. EagleEye is free and without any advertisements. Though I didn't use the app (yet) myself, most of its features (and behaviour) is known to me by its sister-app I'm using. Runs stable, is fun to work with. In short: This one you should definitely give a try :)