There are a few different ways to label appendix in APA, depending on how many appendixes you have and what you are using them for.
If you have just one appendix, you would label it "Appendix." For example:
Appendix A: Detailed Instructions
However, if you have multiple appendixes, you would label them "Appendix A," "Appendix B," and so on. For example:
Appendix A: Detailed Instructions
Appendix B: Data
Appendix C: Sample Questionnaire
If your appendixes include material that would normally be found in a paper (e.g., detailed instructions, data, sample questionnaires, etc.), you would label them as above and refer to them in the text of your paper as needed. For example:
See Appendix A for detailed instructions.
For more information, see the APA Style blog post on how to label and cite appendixes.