Depending on what type of part/assembly, we have multiple guides for editing a part:

• Editing a simple Part: This is for the most basic parts like a washer, channel or a plate which has no moving aspects. Follow the steps in Editing a part with Onshape 
• Editing an Assembly: For a part which must move (like a bearing or a motor). Follow the steps in Editing an Assembly with Onshape
• Combining screws into a part: Some parts have integral screws which belong in the part but don’t ever get removed (such as a set screw or a clamp screw).  For simplicity, they are merged into the part so that they are handled as a single unit. Follow the steps in Editing a Part which includes Screws in Onshape
• Creating a configurable part document: Some parts have multiple variants that a user may want to change without having to replace the part (such as a Channel that could be lengthened).  Once the template is created, it is also useful to create individual documents for each configured part. Follow the steps in Creating a configurable template in Onshape
• Creating the first configured document from a part document: When creating a configurable part (such as sizable channel or selectable gear) it is also important to create an individual document for each variant of the part.  The first step is to create one configured document that gets copied to create the other documents. Follow the steps in Creating the first configured part from a template part
• Creating Multiple Parts from a first configured part: Once the first one is created, generating the others is a much simpler process. Follow the steps in Creating a part from a configurable part