When you are an assistant to of one your colleagues, you need to switch to their SRXP environment to help them submit and/or approve and/or export.
I work with the app:
You can switch to the SRXP account of your colleague with the mobile application when he/she has the submit rights. Click on the stripes first, so the menu will open from the left side. You can switch to the other account as followed (left image for Android, right image for Iphone):
I work with a browser:
You can switch to click on the drop down menu you will find on the top left side under the logo from your company. You can do this via the browser, no matter which rights the other account has: