You have to remove the intake manifold to replace the injectors.
The main activities are these:
1. Remove air filter and housing
2. Remove all cables around the intake plenum
3. Remove throttle body and cables
4. Remove plenum
5. Remove intake manifold
6. Remove injectors from intake manifold
7. This is good time to remove gunk and carbon from intake manifold
7a. This is also a good time to replace the valve cover gaskets
7b. This is also a good time to replace PCV valve
7c. This is also a good time to replace gasket of EGR valve
7d. This is also a good time to replace all vacuum hoses to EGR, plenum, etc
7e. This is also a good time to remove carbon residues from throttle body and plenum
8. Install new injectors with new rubber gaskets into intake manifold
9. Install instake manifold with new gaskets
10. Install intake plenum with new gaskets
11. Install throttle body with new gasket
12. Install air filter with new filter
I replaced the intake manifold gaskets a year ago and took me two days to complete the job. Of course, I took extra time to clean each part before re-assembling.
Just a comment: my van has 193K+ miles and I have never replace an injector.