In most fields, it is an acceptable practice to present published research papers at a conference. However, when presenting a work that has already been published or accepted, there might be some restrictions on how you can reuse the material that has been published/accepted.
For instance, you may need to provide citations when including figures from the paper in your presentation, as well as citing any text that is taken directly from what has been published. For many journals, however, you may do so without asking prior permission, so long as you include the appropriate citations.
If your paper is accepted by the journal, you can write to the journal editor and inform him/her that you are presenting the findings at the conference. Mention that you are presenting only part of the study and not the content in its entirety.
Typically, the journal article should contain at least 30% original material that has not been presented before. The journal might impose some restrictions on how you can present the study at the conference. For instance, you may need to provide citations when including figures from the paper in your presentation, as well as citing any text that is taken directly from what has been published.
Also, ask the journal editor if it is okay to publish the abstract in the conference proceedings. If the journal editor does not agree to it, you can inform the conference organizers that your paper on the same topic has already been accepted by a journal and hence you would not want to include the abstract in the conference proceedings.
You should also check the conference guidelines and and see if they have any specific requirements with regard to presenting an accepted or published paper.