If you're already using Forge Gradle, you can replace your build.gradle file with the one included in the recommended 1.7.10 source download for Forge, fill out the version, group and archivesBaseName properties again and add back any custom logic you had.
After that, you should be able to run gradlew setupDecompWorkspace (eclipse/idea) (using the appropriate IDE name without the parentheses) to set up your workspace for 1.7.10 and regenerate the IDE project files.
If you're not already using it, you can move your code/assets into the appropriate folder structure (see the example mod included in the Forge source download for reference) and then follow the same instructions as above.
Please don't PM me to ask for help or to join your mod development team. Asking your question in a public thread preserves it for people who are having the same problem in the future. I'm not interested in developing mods with people.
You misunderstand. Install Forge for 1.7.10, set it all up like you're starting from scratch, then close your IDE (Eclipse), copy all your code/assets to the new project folder, and re-load eclipse. There will be a *lot* of errors, because syntax and methods have changed.
For assets, the folder structure has changed a bit, and you also must use language registry and lang files from here. I don't have an example to give (my mod is 1.7.10 but I've never tested it in a non development client yet) but there is plenty of repo's for 1.7 out there that show this easily.
The link I provided covers all the basic changes. Once you move your code into a Forge 1.7.10 workspace and update the relevant code following Wuppy's tutorials, come back to us with a specific question - An answer for "How to update" has been provided.
Okay, to be clear, updating to 1.7.x or 1.8 requires four steps:
1) copy all your code into new workspace
2) update Forge as explained by Choonster above
3) import the project into Eclipse
4) Find all the errors flagged by Eclipse and figure out how to fix them.
To understand step 4 you need to realize that in each version there are some methods that change names (some have same functionality but are named differently) and in other cases the code changes the approach (for example in 1.8 they have changed to entirely new system of block and item models). So this means that your 1.6.4 code that didn't have errors, will now have errors that need to be fixed.
As offered above, once you have specific errors in your code identified, you can post back here for help.