I have two DEMs, the first one is in 30 meters of résolution and the second: 5 meters, and will like if there is a possibility of merging them to extend the area where I do my research?
I can think of two options.
Mosaic the two rasters to a new raster dataset, using the Mosaic to New Raster tool or the Workspace to Raster Dataset tool. If you do, you need to choose a single cell size, which means you'll be downsampling the 5-meter DEM or upsampling the 30-meter DEM. (You should add some metadata to the mosaicked raster to explain what you did, especially if you upsample the 30-meter DEM by choosing a cell size of 5 meters. You don't want to make anyone think that the whole raster was originally 5-meter data.)
Create a mosaic dataset. That way you're not resampling anything. ArcGIS analysis tools will treat a mosaic dataset as if it were a single raster dataset.
I prefer #2, since it preserves all the original data.