Upgrade RAM or other ways to improve computer? I have an Aspire A3SP14-31PT.

You cannot upgrade the RAM because , the laptop comes only with 4GB or with 8GB RAM. You can upgrade the SSD to 512GB or beyond. There is no real limit to the drive size and the 1TB and 2TB drives are much more cost effective.

To improve performance follow below guide completely-

(You can skip Gpu driver reinstallation )

As for SSD Upgrade, You can have PCIE Gen3 8Gb/S up to 2 lanes, Nvme

For the Acer Aspire 3 Spin 14 (model A3SP14-31PT) which supports PCIe Gen3 with 2 lanes, I recommend the following SSD options:

1. Crucial P3 500GB PCIe 3.0 NVMe M.2 SSD - Offers excellent read/write speeds of up to 3500/3000 MB/s, significantly faster than the original SSD. It will provide a noticeable performance boost while being compatible with the laptop's PCIe Gen3 x2 interface.
2. WD Black SN750 500GB NVMe SSD - A reliable and fast option that can fully utilize the PCIe Gen3 x2 interface, delivering sequential read/write speeds up to 3,430/3,000 MB/s. It features advanced thermal management for consistent high performance.
3. Samsung 970 EVO Plus 500GB NVMe SSD - Known for its high performance and reliability, the 970 EVO Plus is a great choice for demanding applications. While it supports PCIe Gen3 x4, it will still provide excellent speeds when limited to x2 lanes.

These SSDs will significantly improve boot times, application loading, and overall system responsiveness compared to the original 128GB SSD. The Crucial P3 and WD Black SN750 are the most cost-effective options that can fully utilize the available PCIe Gen3 x2 interface on the Acer Aspire 3 Spin 14.

You can get higher storage versio of these ssd